enic: Add support to configure hardware interrupt coalesce timers in a platform independent way

enic driver and the underlying hardware use different units for representing the interrupt coalesce timer.
Driver converts the interrupt coalesce timer in usec to hardware cycles while setting the relevant hardware
registers. The conversion factor can be different for each of the adapter hardware types. So it is dynamically
learnt from the adapter firmware using the devcmd CMD_INTR_COAL_CONVERT. This allows the driver to configure
the hardware interrupt coalesce timers in a platform independent way.

Signed-off-by: Danny Guo <dannguo@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: David Wang <dwang2@cisco.com>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>