bnxt_en: Refactor and simplify coalescing code.

The mapping of the ethtool coalescing parameters to hardware parameters
is now done in bnxt_hwrm_set_coal_params().  The same function can
handle both RX and TX settings.  The code is now more clear.  Some
adjustments have been made to get better hardware settings.  The
coal_frames setting is now accurately set in hardware.  The max_timer
is set to coal_ticks value.

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