openvswitch: rework skipping intree kmod build for unsupported kernels

openvswitch: rework skipping intree kmod build for unsupported kernels

This is a workaround to prevent the whole build from failing because of
the intree kmods are not supported yet by upstream project.

Root cause is that kernel version should not play a part when making
DEPENDS as the generated kconfig was for all targets that may have
different kernel versions.

One less than ideal effect of this change is that for an unsupported
kernel version, people can still select the intree kmod but it won't be
built.  This may contradict expectation if the warning was not noticed
by them

Resolves openwrt/packages#9274

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>