wil6210: ethtool ops

Provide ethtool support; with support for interrupt coalescing through
get_coalesce/set_coalesce.
Placeholders for begin/complete will be used by runtime PM
to make sure target is powered up while performing ethtool operations

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>