staging: rtl8192e: rtl_dm: Make functions static

Make as many functions as possible static in rtllib_dm.c.

The following functions were affected:
- dm_check_fsync
- dm_CheckRfCtrlGPIO
- dm_fsync_timer_callback

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>