usb: dwc3: document gfladj_refclk_lpm_sel field

Avoid a kernel-doc warning by documenting it:
	drivers/usb/dwc3/core.h:1343: warning:
	  Function parameter or member 'gfladj_refclk_lpm_sel'
          not described in 'dwc3'

Fixes: a6fc2f1b

 ("usb: dwc3: core: add gfladj_refclk_lpm_sel quirk")
Signed-off-by: Swarup Laxman Kotiaklapudi <swarupkotikalapudi@gmail.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20231002210137.209382-1-swarupkotikalapudi@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>