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

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

BugLink: https://bugs.launchpad.net/bugs/1959879
commit 945c37ed564770c78dfe6b9f08bed57a1b4e60ef upstream.

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

Fixes: c6919d5e0cd1 ("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>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>