libata: fix hw_sata_spd_limit initialization

hw_sata_spd_limit used to be incorrectly initialized to zero instead
of UINT_MAX if SPD is zero in SControl register.  This breaks PHY
speed down.  Fix it.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>