hw/char/serial: Clean up unnecessary code

Since commit 5ec3a23 ("serial: convert PIO to new memory
api read/write") we don't need to worry about accesses bigger
than 8-bit. Use the extract()/deposit() functions to access
the correct part of the 16-bit 'divider' register.

Reported-by: Jonathan D. Belanger <jbelanger1@rochester.rr.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1904331
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201120161933.2514089-1-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/char/serial.c

@@ -24,6 +24,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/bitops.h"
 #include "hw/char/serial.h"
 #include "hw/irq.h"
 #include "migration/vmstate.h"
@@ -338,11 +339,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val,
     default:
     case 0:
         if (s->lcr & UART_LCR_DLAB) {
-            if (size == 1) {
-                s->divider = (s->divider & 0xff00) | val;
-            } else {
-                s->divider = val;
-            }
+            s->divider = deposit32(s->divider, 8 * addr, 8, val);
             serial_update_parameters(s);
         } else {
             s->thr = (uint8_t) val;
@@ -364,7 +361,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val,
         break;
     case 1:
         if (s->lcr & UART_LCR_DLAB) {
-            s->divider = (s->divider & 0x00ff) | (val << 8);
+            s->divider = deposit32(s->divider, 8 * addr, 8, val);
             serial_update_parameters(s);
         } else {
             uint8_t changed = (s->ier ^ val) & 0x0f;
@@ -478,7 +475,7 @@ static uint64_t serial_ioport_read(void *opaque, hwaddr addr, unsigned size)
     default:
     case 0:
         if (s->lcr & UART_LCR_DLAB) {
-            ret = s->divider & 0xff;
+            ret = extract16(s->divider, 8 * addr, 8);
         } else {
             if(s->fcr & UART_FCR_FE) {
                 ret = fifo8_is_empty(&s->recv_fifo) ?
@@ -502,7 +499,7 @@ static uint64_t serial_ioport_read(void *opaque, hwaddr addr, unsigned size)
         break;
     case 1:
         if (s->lcr & UART_LCR_DLAB) {
-            ret = (s->divider >> 8) & 0xff;
+            ret = extract16(s->divider, 8 * addr, 8);
         } else {
             ret = s->ier;
         }