xhci: dbgtty: Make some functions static

xhci: dbgtty: Make some functions static

The sparse tool complains as follows:

drivers/usb/host/xhci-dbgtty.c:401:5: warning:
 symbol 'xhci_dbc_tty_register_device' was not declared. Should it be static?
drivers/usb/host/xhci-dbgtty.c:452:6: warning:
 symbol 'xhci_dbc_tty_unregister_device' was not declared. Should it be static?

After commit 6ae6470bfa33 ("xhci: dbc: Add a operations structure
to access driver functions"), those functions are not used outside
of xhci-dbgtty.c, so this commit marks them static.

Fixes: 6ae6470bfa33 ("xhci: dbc: Add a operations structure to access driver functions")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20200727171149.3011-1-weiyongjun1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>