selftests: firmware: skip unsupported custom firmware fallback tests

Ignore custom firmware loading and cancellation tests on older
kernel releases, which do not support this feature.

Fixes: 061132d2

 ("test_firmware: add test custom fallback trigger")
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>