usb: mtu3: remove mtu3_ep0_setup() declaration in mtu3.h

It's defined and only used in the same file, so remove its declaration
in mtu3.h, and make it static

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1622182260-23767-1-git-send-email-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>