usb: mtu3: declare functions static

The only user of the  functions mtu3_irq() and gpd_ring_empty() are in
the same file. They can be declared as static.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>