[SCSI] fix locking in host use of blk_plug_device()

scsi_lib.c:scsi_host_queue_ready() plugs the device with incorrect
locking.  It should actually have the queue lock held, but it's
holding the host lock.  Fix this by eliminating the call.  The host
ready has no need to plug the queue because if it returns 0 in
scsi_request_function control transfers to not_ready which acquires
the queue lock and plugs the device if its at zero depth.

Reported-by: Elias Oltmanns <eo@nebensachen.de>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>