IB/usnic: Simplify the code to balance loc/unlock calls

Simplify code in find_free_vf_and_create_qp_grp() to avoid sparse error
regarding call to unlock in the block other than lock was called.

drivers/infiniband/hw/usnic/usnic_ib_verbs.c:206:9: warning: context imbalance
			in 'find_free_vf_and_create_qp_grp' - different lock
			contexts for basic block

CC: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>