staging/rtl8188eu/os_dep: Remove unneeded variable ret

Below list of functions returns 0 in success and -EINVAL in failure. So
directly return 0 on Success.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>