libata: increase the timeout when setting transfer mode

I have a ST4000DM000 disk. If Linux is booted while the disk is spun down,
the command that sets transfer mode causes the disk to spin up. The
spin-up takes longer than the default 5s timeout, so the command fails and
timeout is reported.

Fix this by increasing the timeout to 15s, which is enough for the disk to
spin up.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>