rtl8192u: replace __FUNCTION__ with __func__ in r8192U_core.c

This patch fixes the following checkpatch warning:
WARNING: __func__ should be used instead of gcc
specific __FUNCTION__

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>