selftests: uevent filtering: fix return on error in uevent_listener

The ret variable is used to check function return values and assigning
values to it on error has no effect as it is an unused value.

The current implementation uses an additional variable (fret) to return
the error value, which in this case is unnecessary and lead to the above
described misuse. There is no restriction in the current implementation
to always return -1 on error and the actual negative error value can be
returned safely without storing -1 in a specific variable.

Simplify the error checking by using a single variable which always
holds the returned value.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>