staging: r8188eu: Fix sparse warning in core/rtw_sta_mgt.c

Sparse reports the following:

  CHECK   drivers/staging/rtl8188eu/core/rtw_sta_mgt.c
drivers/staging/rtl8188eu/core/rtw_sta_mgt.c:152:6: warning: symbol 'rtw_mfree_all_stainfo' was not declared. Should it be static?

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>