staging: dwc2: remove compile warning for USB_DWC2_TRACK_MISSED_SOFS

Remove the compile-time warning for this config option, and instead
warn that it is experimental in the Kconfig text

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>