staging: r8188eu: Fix sparse warnings regarding static functions

This fixes the following sparse warnings:
* drivers/staging/rtl8188eu/core/rtw_cmd.c:221:5: warning:
symbol 'rtw_cmd_filter' was not declared. Should it be static?
* drivers/staging/rtl8188eu/core/rtw_cmd.c:1670:6: warning:
symbol 'dynamic_chk_wk_hdl' was not declared. Should it be static?

Signed-off-by: Valentina Manea <valentina.manea.m@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>