spi/bfin_spi: use nosync when disabling the IRQ from the IRQ handler

Using disable_irq() on the IRQ whose handler we are currently executing in
can easily lead to a hang.  So use the nosync variant here.

Signed-off-by: Yi Li <yi.li@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>