spi: xcomm: Remove duplicate code to set default bits_per_word and max speed

In the implementation of __spi_validate(), spi core will set transfer
bits_per_word and max speed as spi device default if it is not set for
this transfer. So we can remove the same logic in spi_xcomm_setup_transfer().
Also remove a redundant code to initialize is_first variable.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>