Staging: rtl8192u: fix functions that should not be declared extern

These functions are already marked extern in the header file

drivers/staging/rtl8192u/r819xU_phy.c:1716:13: warning: function 'InitialGainOperateWorkItemCallBack' with external linkage has definition
drivers/staging/rtl8192u/r819xU_cmdpkt.c:497:12: warning: function 'cmpk_message_handle_rx' with external linkage has definition

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>