wiringpi: fix compile for missing baud defines

wiringpi: fix compile for missing baud defines

For sparc-buildroot-linux-uclibc/sysroot/usr/include/asm/termbits.h
containing:

  /* These have totally bogus values and nobody uses them
     so far. Later on we'd have to use say 0x10000x and
     adjust CBAUD constant and drivers accordingly.
  #define B2500000  0x00001010
  #define B3000000  0x00001011
  #define B3500000  0x00001012
  #define B4000000  0x00001013  */

Fixes [1]:

  wiringSerial.c: In function 'serialOpen':
  wiringSerial.c:78:28: error: 'B2500000' undeclared (first use in this function)
       case 2500000: myBaud = B2500000 ; break ;
                              ^
  wiringSerial.c:78:28: note: each undeclared identifier is reported only once for each function it appears in
  wiringSerial.c:79:28: error: 'B3000000' undeclared (first use in this function)
       case 3000000: myBaud = B3000000 ; break ;
                              ^
  wiringSerial.c:80:28: error: 'B3500000' undeclared (first use in this function)
       case 3500000: myBaud = B3500000 ; break ;
                              ^
  wiringSerial.c:81:28: error: 'B4000000' undeclared (first use in this function)
       case 4000000: myBaud = B4000000 ; break ;
                              ^

[1] http://autobuild.buildroot.net/results/5be/5bea1e95d1c7b444a5f0a5770251f1af3efc9c18

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>