Fix build on SPARC

On SPARC, the definitions of B2500000, B3000000, B3500000 and B4000000
are not necessarily available, so use those values only if defined in
the kernel headers.

It fixes SPARC build failures such as:

src/serial.c: In function '_serial_baudrate_to_bits':
src/serial.c:73:30: error: 'B2500000' undeclared (first use in this function)
         case 2500000: return B2500000;
                              ^
src/serial.c:73:30: note: each undeclared identifier is reported only once for each function it appears in
src/serial.c:74:30: error: 'B3000000' undeclared (first use in this function)
         case 3000000: return B3000000;
                              ^

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

src/serial.c

@@ -70,10 +70,18 @@ static int _serial_baudrate_to_bits(uint32_t baudrate) {
         case 1152000: return B1152000;
         case 1500000: return B1500000;
         case 2000000: return B2000000;
+#ifdef B2500000
         case 2500000: return B2500000;
+#endif
+#ifdef B3000000
         case 3000000: return B3000000;
+#endif
+#ifdef B3500000
         case 3500000: return B3500000;
+#endif
+#ifdef B4000000
         case 4000000: return B4000000;
+#endif
         default: return -1;
     }
 }
@@ -107,10 +115,18 @@ static int _serial_bits_to_baudrate(uint32_t bits) {
         case B1152000: return 1152000;
         case B1500000: return 1500000;
         case B2000000: return 2000000;
+#ifdef B2500000
         case B2500000: return 2500000;
+#endif
+#ifdef B3000000
         case B3000000: return 3000000;
+#endif
+#ifdef B3500000
         case B3500000: return 3500000;
+#endif
+#ifdef B4000000
         case B4000000: return 4000000;
+#endif
         default: return -1;
     }
 }