Fix SPI: IOError: [Errno 22] Invalid argument in xfer and xfer2 funct…

[kozga]

Fix for #115

[pdp7]

@kozga FYI - test after new build from master:

root@beaglebone:~# cat xfer2test.py 
from Adafruit_BBIO.SPI import SPI        

spi = SPI(1, 0)                          
spi.mode = 0                             
spi.msh = 500000                         

msg = spi.xfer2([0xd7, 0x00])            
print ' '.join('0x%02x' % b for b in msg)

root@beaglebone:~# python xfer2test.py
0x00 0x00