MAX7301 GPIO: Do not force SPI speed when using OF Platform

The bit_per_word can be set in the OF Device tree, so no need to force it as with
the platform_data when using OF Platform

Signed-off-by: Patrick Vasseur <patrick.vasseur@c-s.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>