spi: octeon: Convert to use bits_per_word_mask

spi: octeon: Convert to use bits_per_word_mask

Since commit 543bb25 "spi: add ability to validate xfer->bits_per_word in SPI
core", the driver can set bits_per_word_mask for the master then the SPI core
will reject transfers that attempt to use an unsupported bits_per_word value.
So we can remove octeon_spi_validate_bpw() and let SPI core handle the checking.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>