Staging: rtl8192 Clean up function definition

Change function definition to match its prototype declaration. This
fixes the following warning generated by sparse:

drivers/staging/rtl8192u/r8192U_core.c:1970:6: warning: symbol
'rtl8192_update_ratr_table' was not declared. Should it be static?

Signed-off-by: Eddie Kovsky <ewk@edkovsky.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>