staging/rtl8192u: fix defined but not used build warnings

staging/rtl8192u: fix defined but not used build warnings

Fix build warnings in rtl8192u when CONFIG_PROC_FS is not enabled
by marking the unused functions as __maybe_unused.

../drivers/staging/rtl8192u/r8192U_core.c:508:12: warning: 'proc_get_stats_ap' defined but not used [-Wunused-function]
../drivers/staging/rtl8192u/r8192U_core.c:527:12: warning: 'proc_get_registers' defined but not used [-Wunused-function]
../drivers/staging/rtl8192u/r8192U_core.c:568:12: warning: 'proc_get_stats_tx' defined but not used [-Wunused-function]
../drivers/staging/rtl8192u/r8192U_core.c:627:12: warning: 'proc_get_stats_rx' defined but not used [-Wunused-function]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jerry chuang <wlanfae@realtek.com>
Cc: devel@driverdev.osuosl.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>