bnxt_en: Refactor bnxt_hwrm_set_coal().

Add a function to set all the coalescing parameters.  The function can
be used later to set both rx and tx coalescing parameters.

v2: Fixed function parameters formatting requested by DaveM.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>