ide-cd: add missing 'ireason' masking to cdrom_write_intr()

Mask 'ireason' variable with 0x3 so the valid interrupt reason value
is passed to cdrom_write_check_ireason() for checking.

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