ata: pata_ali: Supply description for 'ali_20_filter()'s 'mask' param

Fixes the following W=1 kernel build warning(s):

 drivers/ata/pata_ali.c:119: warning: Function parameter or member 'mask' not described in 'ali_20_filter'

Cc: Jens Axboe <axboe@kernel.dk>
Cc: CJ <cjtsai@ali.com.tw>
Cc: Andre Hedrick <andre@linux-ide.org>
Cc: Alan Cox <alan@redhat.com>
Cc: Clear Zhang <Clear.Zhang@ali.com.tw>
Cc: linux-ide@vger.kernel.org
Cc: and cc <htejun@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>