ath9k: Re-enable RXOEL interrupt after processing rx buffers

Once RXEOL was disabled, it never be enabled again. This patch
re-enables rxeol at the end of rx tasklet.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>