selftests: firmware: skip unsupported async loading tests

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

Fixes: 1b1fe542:
       ("selftests: firmware: add empty string and async tests")
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>