usb: gadget: add '__ref' for rndis_config_register() and cdc_config_register()

They are only called by '__ref' function multi_bind(), and they will
call '__init' functions, so recommend to let them '__ref' too.

The related warnings:

  WARNING: drivers/usb/gadget/g_multi.o(.text+0xded6): Section mismatch in reference from the variable .LM2921 to the variable .init.text:_rndis_do_config
  The function .LM2921() references
  the variable __init _rndis_do_config.
  This is often because .LM2921 lacks a __init
  annotation or the annotation of _rndis_do_config is wrong.

  WARNING: drivers/usb/gadget/g_multi.o(.text+0xdf16): Section mismatch in reference from the variable .LM2953 to the variable .init.text:_cdc_do_config
  The function .LM2953() references
  the variable __init _cdc_do_config.
  This is often because .LM2953 lacks a __init
  annotation or the annotation of _cdc_do_config is wrong.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>