libata: fix error checking in in ata_parse_force_one()

ata_parse_force_one() was incorrectly comparing @p to @endp when it
should have been comparing @id.  The only consequence is that it may
end up using an invalid port number in "libata.force" module param
instead of rejecting it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Petru-Florin Mihancea <petrum@gmail.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=195785