scsi: qla2xxx: Fix crash due to NULL pointer dereference of ctx

Fixes following signature in the stack trace:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000374
IP: [<ffffffffa06ec8eb>] qla2x00_sp_free_dma+0xeb/0x2a0 [qla2xxx]

Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Joe Carnuccio <joe.carnuccio@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>