hw/i386/pc: fix code comment on cumulative flash size

- The comment is incorrectly indented / formatted.

- The comment states a 8MB limit, even though the code enforces a 16MB
  limit.

Both of these warts come from commit 0657c65 ("hw/i386/pc: add max
combined fw size as machine configuration option", 2020-12-09); clean them
up.

Arguably, it's also better to be consistent with the binary units (such as
"MiB") that QEMU uses nowadays.

Cc: "Michael S. Tsirkin" <mst@redhat.com> (supporter:PC)
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com> (supporter:PC)
Cc: Paolo Bonzini <pbonzini@redhat.com> (maintainer:X86 TCG CPUs)
Cc: Richard Henderson <richard.henderson@linaro.org> (maintainer:X86 TCG CPUs)
Cc: Eduardo Habkost <eduardo@habkost.net> (maintainer:X86 TCG CPUs)
Cc: qemu-trivial@nongnu.org
Fixes: 0657c65
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/i386/pc.c

@@ -1746,12 +1746,12 @@ static void pc_machine_set_max_fw_size(Object *obj, Visitor *v,
     }
 
     /*
-    * We don't have a theoretically justifiable exact lower bound on the base
-    * address of any flash mapping. In practice, the IO-APIC MMIO range is
-    * [0xFEE00000..0xFEE01000] -- see IO_APIC_DEFAULT_ADDRESS --, leaving free
-    * only 18MB-4KB below 4G. For now, restrict the cumulative mapping to 8MB in
-    * size.
-    */
+     * We don't have a theoretically justifiable exact lower bound on the base
+     * address of any flash mapping. In practice, the IO-APIC MMIO range is
+     * [0xFEE00000..0xFEE01000] -- see IO_APIC_DEFAULT_ADDRESS --, leaving free
+     * only 18MiB-4KiB below 4GiB. For now, restrict the cumulative mapping to
+     * 16MiB in size.
+     */
     if (value > 16 * MiB) {
         error_setg(errp,
                    "User specified max allowed firmware size %" PRIu64 " is "