scsi: iscsi: Use EFI GetVariable only when available

Replace the EFI runtime services check with one that tells us whether
EFI GetVariable() is implemented by the firmware.

Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>