ar7-atm: drop LINUX_VERSION tests

ar7-atm: drop LINUX_VERSION tests

Minimum supported kernel is 3.18, so we don't need to test for anything
older. In addition, the API hasn't changed since then, so we don't need
to check for any kernel version at all.  This helps to keeps the amount
of changes more managable.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>