spi/bfin_spi: work around anomaly 05000119

Anomaly 05000119 states that the DMA_RUN bit with peripherals isn't
reliable.  However, the way the driver is currently written (DMA IRQ
callback), we don't need the polling in the first place, so drop it.

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