USB: musb: Refer to musb_otg_timer_func under correct #ifdef

musb_otg_timer_func() is defined under #ifdef CONFIG_USB_MUSB_OTG.
Make sure any reference to it is also under the same #ifdef.

Without this fix, the driver failes to compile when USB_OTG is defined
but USB_MUSB_OTG isn't.

Signed-off-by: Amit Kucheria <amit.kucheria@canonical.com>
Cc: Felipe Balbi <felipe.balbi@nokia.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>