spi/bfin_spi: warn when CS is driven by hardware (CPHA=0)

When the hardware is controlling the CS, there are some SPI options
we are unable to support.  So issue a warning in the hopes that the
user will change to a SPI mode where we can support things sanely.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>