scsi: libsas: Export sas_phy_enable()

Export sas_phy_enable() so LLDDs can directly use it to control remote
phys.

We already do this for companion function sas_phy_reset().

Link: https://lore.kernel.org/r/1634041588-74824-4-git-send-email-john.garry@huawei.com


Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>