enic: allow adaptive coalesce setting for msi/legacy intr

* Allow setting of adaptive coalescing setting for all types of interrupt.

* In msi & legacy intr, we use single interrupt for rx & tx. In this case
  tx_coalesce_usecs is invalid. We should use only rx_coalesce_usecs.
  Do not display tx_coal values for msi/intx. And do not allow user to set
  this as well.

* Driver supports only tx/rx_coalesce_usec and adaptive coalesce settings.
  For other values, driver does not return error. So ethtool succeeds for
  unsupported values. Introduce enic_coalesce_valid() function to validate
  the coalescing values.

* If user requests for coalesce value greater than what adaptor supports,
  driver uses the max value. We should at least log this.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>