staging: gdm724x: Fix sparse warnings regarding static functions/variables

This fixes sparse warnings for functions and variables, e.g.:
* drivers/staging/gdm724x/gdm_mux.c:29:25: warning: symbol
'mux_rx_wq' was not declared. Should it be static?

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