xhci: dbc: Make function xhci_dbc_ring_alloc() static

xhci: dbc: Make function xhci_dbc_ring_alloc() static

The sparse tool complains as follows:

drivers/usb/host/xhci-dbgcap.c:422:18: warning:
 symbol 'xhci_dbc_ring_alloc' was not declared. Should it be static?

This function is not used outside ofxhci-dbgcap.c, so this commit
marks it static.

Fixes: ac286428c69f ("xhci: dbc: don't use generic xhci ring allocation functions for dbc.")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20200727171207.3101-1-weiyongjun1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>