staging: rtl8188eu: declare internal function as static

This function is only used inside rtw_recv.c. Which is quite logical,
since it's a timer callback: it is passed as the _fn argument to
setup_timer(). So it is internal to rtw_recv.c and should be static.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Geliang Tang <geliangtang@163.com>
Cc: Jakub Sitnicki <jsitnicki@gmail.com>
Cc: Ivan Safonov <insafonov@gmail.com>
Cc: Kyle Kuffermann <kyle.kuffermann@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>