spi: core: Only check bits_per_word validity when explicitly provided

On SPI device probe, the core will call spi_setup in spi_add_device
before the corresponding driver was probed. When this happens, the
bits_per_word member of the device is not yet set by the driver,
resulting in the default being set to 8 bits-per-word.

However some controllers do not support 8 bits-per-word at all, which
results in a failure when checking the bits-per-word validity.

In order to support these devices, skip the bits-per-word validity
check when it is not explicitly provided by drivers.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://lore.kernel.org/r/20220412122207.130181-1-paul.kocialkowski@bootlin.com


Signed-off-by: Mark Brown <broonie@kernel.org>