usb: roles: fix include/linux/usb/role.h compile issue

when CONFIG_USB_ROLE_SWITCH is not defined,
add usb_role_switch_find_by_fwnode() definition which return NULL.

Fixes: c6919d5e

 ("usb: roles: Add usb_role_switch_find_by_fwnode()")
Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/1641818608-25039-1-git-send-email-quic_linyyuan@quicinc.com


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