spi/bfin_spi: reset ctl_reg bits when setup is run again on a device

During runtime, the spi setup function may be called multiple times on the
same device in order to reconfigure some settings on the fly.  When this
happens, we need to reset the ctl_reg bits so that changing the mode works
as expected.

Reported-by: Andy Getzendanner <james.getzendanner@students.olin.edu>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>