During the creation of a new spi device, we need to
have the possiblility to give the bits_per_word value
to master->setup.

As a matter of fact, spi master could check the spi
device bits_per_word and compare this value against its
capabilities.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>