SPI100k: Fix 8-bit and RX-only transfers

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: Cory Maccarrone <darkstar6262@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>