spi: add flags parameter to txrx_word function pointers

Add the capability to specify the flag parameter used in
bitbang_txrx_be_cpha{0,1} through the txrx_word function pointers of
spi_bitbang data structure. That feature will be used to add spi-3wire
support to the spi-gpio controller

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Mark Brown <broonie@kernel.org>