staging: rtl8712: Declare function as static

Declare rtl871x_intf_resume() function static since it is defined and called
in this file only.

This fixes the following sparse warning:
warning: symbol 'rtl871x_intf_resume' was not declared. Should it be static?

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>