Fix the following gcc warning:

drivers/staging/rtl8723bs/core/rtw_mlme.c:2920:6: warning: variable
‘pframe’ set but not used [-Wunused-but-set-variable]
  u8 *pframe;
      ^~~~~~
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200427032342.27211-8-yanaijie@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>