staging: rtl8188eu: fix potential leak in rtw_wx_read32()

Function rtw_wx_read32() dynamically allocates a temporary buffer that is not
freed in all error paths. Use a centralized exit path and make sure that all
memory is freed correctly. Detected by Coverity - CID 1077711.

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