cmd64x: don't clear the other channel's interrupt

Make sure to not clear the other IDE channel's interrupt when clearing an IDE
interrupt via the MRDMODE register.

Thanks to Bart for finding a coding mistake.

Bart:

This fixes regression from commit 66602c83
("cmd64x: use interrupt status from MRDMODE register (take 2)").

Extra thanks to Martin for reporting and bisecting the issue.

From: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Tested-by: Martin Rogge <marogge@onlinehome.de>
Tested-by: Milan Kocian <milon@wq.cz>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>