spi/bitbang: initialize bits_per_word as specified by spi message

SPI protocol drivers can submit messages specifying needed bits_per_word
parameter for a message transfer. The bitbang driver currently ignores
bits_per_word given by a singe message and always uses master's
bits_per_word parameter. Only use master's bits_per_word when a
message didn't specify needed bits_per_word for ongoing transfer.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>