ata: libata-pmp: Fix misspelling of 'val'

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

 drivers/ata/libata-pmp.c:76: warning: Function parameter or member 'val' not described in 'sata_pmp_write'
 drivers/ata/libata-pmp.c:76: warning: Excess function parameter 'r_val' description in 'sata_pmp_write'

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Tejun Heo <teheo@suse.de>
Cc: linux-ide@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>