libata: Add helper to determine when PHY events should be ignored

commit 8393b811f38acdf7fd8da2028708edad3e68ce1f upstream.

This is a preparation commit that will allow to add other criteria
according to which PHY events should be dropped.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>