ide-cd: fix 'ireason' reporting in cdrom_pc_intr()

Mask 'ireason' variable so only the valid interrupt reason bits
will be reported on "drive appears confused" error.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>