spi/pl022: strengthen FIFO watermark level checks

The platform configuration can select custom FIFO watermarks, but
these may conflict the actual FIFO size of the PL022 variant if
set too high. So strengthen the sanity checks to deny any
conflicting settings.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>