spi/bfin_spi: use dma_disable_irq_nosync() in 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: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>