mmc: usdhi6rol0: fix ack register write

The intent appears to be to clear only the bits which are set in status
(by setting them to zero in the ack write), like in the other interrupt
handlers, and not to always clear everything (by always writing zero).
Use the correct not operator.

Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>