mfd: omap-usb-tll: File headers are not good candidates for kernel-doc

Fixes the following W=1 kernel build warning(s):

 drivers/mfd/omap-usb-tll.c:24: warning: expecting prototype for omap(). Prototype was for USBTLL_DRIVER_NAME() instead

Cc: Tony Lindgren <tony@atomide.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Keshava Munegowda <keshava_mgowda@ti.com>
Cc: Roger Quadros <rogerq@ti.com>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>