Prevent OTG peripheral trying to change to b_wait_acon if after
file storage CBW stall followed by SUSPEND from host. We switch
back to b_peripheral if RESET in b_wait_acon mode.

Also fix compile warning for missing default case.

And don't try to set b_wait_acon mode unless CONFIG_USB_OTG.
Signed-off-by: Tony Lindgren <tony@atomide.com>