test_firmware: fix setting old custom fw path back on exit

The file /sys/module/firmware_class/parameters/path can be used
to set a custom firmware path. The fw_filesystem.sh script creates
a temporary directory to add a test firmware file to be used during
testing, in order for this to work it uses the custom path syfs file
and it was supposed to reset back the file on execution exit. The
script failed to do this due to a typo, it was using OLD_PATH instead
of OLD_FWPATH, since its inception since v3.17.

Its not as easy to just keep the old setting, it turns out that
resetting an empty setting won't actually do what we want, we need
to check if it was empty and set an empty space.

Without this we end up having the temporary path always set after
we run these tests.

Fixes: 0a8adf58

 ("test: add firmware_class loader test")
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>