spi/bfin_spi: respect cs_change even in last transfer

According to the definition of flag cs_change in spi.h, CS should stay
active in the last transfer of a message until the first transfer of
the next message if it's set.  The Blackfin SPI driver was not checking
this before deactivating the CS.

Reported-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>