IB/qib: Eliminate divide/mod in converting idx to egr buf pointer

The context init now saves a shift from rcvegrbufs_perchunk
rcvegrbufs_perchunk_shift using ilog2.   A BUG_ON() protects the
power of 2 assumption.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/qib/qib_init.c

@@ -183,6 +183,9 @@ struct qib_ctxtdata *qib_create_ctxtdata(struct qib_pportdata *ppd, u32 ctxt)
 		rcd->rcvegrbuf_chunks = (rcd->rcvegrcnt +
 			rcd->rcvegrbufs_perchunk - 1) /
 			rcd->rcvegrbufs_perchunk;
+		BUG_ON(!is_power_of_2(rcd->rcvegrbufs_perchunk));
+		rcd->rcvegrbufs_perchunk_shift =
+			ilog2(rcd->rcvegrbufs_perchunk);
 	}
 	return rcd;
 }