compat: add compat_version read-only module parameter

compat: add compat_version read-only module parameter

This will allow you to supply a version tag for the
backport you use. Compat-wireless will re-use this and
give it its own value based on the kernel we backport
from. It will use either the linux-next git describe
or the stable kernel git describe. Any delta on top of
the git tree should also be reflected then in case of
custom private trees/releases.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>