bnxt_en: Refactor bnxt_hwrm_set_coal()

Add 2 helper functions to set coalescing for each RX and TX rings.  This
will make it easier to expand the number of TX rings per MSIX in the
next patches.

Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>