usb: gadget: tcm_usb_gadget: mark bot_cleanup_old_alt static

that symbol is only used inside this function driver,
we should mark it static. Fixes sparse's:

drivers/usb/gadget/tcm_usb_gadget.c:373:6: warning: symbol \
	'bot_cleanup_old_alt' was not declared. Should it \
	be static?

Signed-off-by: Felipe Balbi <balbi@ti.com>