RDMA/bnxt_re: Free doorbell page index (DPI) during dealloc ucontext

The driver must free the DPI during the dealloc_ucontext
instead of freeing it during dealloc_pd. However, the DPI
allocation scheme remains unchanged.

Signed-off-by: Devesh Sharma <devesh.sharma@broadcom.com>
Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>