spi: au1550: Simplify au1550_spi_setupxfer()

SPI core validates both bits_per_word and speed_hz transfer parameters and
defaults to spi->bits_per_word and spi->max_speed_hz in case these per
transfer parameters are not set. This can simplify a little the
au1550_spi_setupxfer() as there is need to check only for valid
"struct spi_transfer" pointer.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>