Fix build on SPARC
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 <a5d87d2bd7c424ee5cbdb4e978635bf653d5a005@free-electrons.com>

vsergeev/c-periphery