ath9k: remove useless variable assignment in ath_mci_intr()

Value assigned to variable offset at line 551 is overwritten at line 562,
before it can be used. This makes such variable assignment useless.

Addresses-Coverity-ID: 1226941
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>