bnxt_en: Fix context memory allocation.

When allocating memory pages for context memory, if the last page table
should be fully populated, the current code will set nr_pages to 0 when
calling bnxt_alloc_ctx_mem_blk().  This will cause the last page table
to be completely blank and causing some RDMA failures.

Fix it by setting the last page table's nr_pages to the remainder only
if it is non-zero.

Fixes: 08fe9d18 ("bnxt_en: Add Level 2 context memory paging support.")
Reported-by: Eric Davis <eric.davis@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>