This change fixes 8-bit transfers and RX-only transfers.  The
SPI100k framework requires minimum 16-bit words to be written, so 8-bit
transfers must be shited by 8 bits and sent out as a 16-bit word.

Additionally, receive-only transfers were failing due to the
perceived need to fill the TX buffer with something.  This is in
fact not needed.
Signed-off-by: NCory Maccarrone <darkstar6262@gmail.com>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>