staging: rtl8192u: Deleted 'rtl8192_beacon_stop' function in r8192U_core.c

This patch delete the 'rtl8192_beacon_stop' function that is not used
in anywhere in the driver.

Fix the following sparse warning in r8192U_core.c

drivers/staging/rtl8192u/r8192U_core.c:1332:6: warning: symbol 'rtl8192_beacon_stop' was not declared. Should it be static?

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>