bnxt_en: refactor VNIC RSS update functions

Extract common code into a new function. This will avoid duplication
in the next patch, which changes the update algorithm for both the P5
and legacy code paths.

No functional changes.

Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>