RDMA/qedr: Fix some error handling

'qedr_alloc_pbl_tbl()' can not return NULL.

In qedr_init_user_queue():
 - simplify the test for the return value, no need to test for NULL
 - propagate the error pointer if needed, otherwise 0 (success) is returned.
   This is spurious.

In init_mr_info():
 - test the return value with IS_ERR
 - propagate the error pointer if needed instead of an exlictit -ENOMEM.
   This is a no-op as the only error pointer that we can have here is
   already -ENOMEM

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Ram Amrani <Ram.Amrani@cavium.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>