bnxt_en: Reorganize the coalescing parameters.

The current IRQ coalescing logic is a little messy.  The ethtool
parameters are mapped to hardware parameters in a way that is difficult
to understand.  The first step is to better organize the parameters
by adding the new structure bnxt_coal.  The structure is used by both
the RX and TX sets of coalescing parameters.

Adjust the default coal_ticks to 14 us and 28 us for RX and TX.

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