staging: rtl8188eu: fix potential leak in update_bcn_wps_ie()

Fix a potential leak in the error path of function update_bcn_wps_ie().
Move the affected input verification to the beginning of the function so
that it may return directly without leaking already allocated memory.
Detected by Coverity - CID 1077718.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>