bnxt_en: Update interrupt coalescing logic.

New firmware spec. allows interrupt coalescing parameters, such as
maximums, timer units, supported features to be queried.  Update
the driver to make use of the new call to query these parameters
and provide the legacy defaults if the call is not available.

Replace the hard-coded values with these parameters.

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