spi/bfin_spi: handle error/status changes after data interrupts

The error interrupt on the BF537 SIC cannot be enabled on a
per-peripheral basis.  Once the error interrupt is enabled
for one peripheral, it is automatically enabled for all.

So in the Blackfin on-chip SPI driver, we need to clear out
these known errors in the data interrupt once we've successfully
finished processing all of the pending data.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>