Resolve the following sparse warnings:

arch/arm/mach-omap1/usb.c:304:12: warning: symbol 'omap1_usb0_init' was not declared. Should it be static?
arch/arm/mach-omap1/usb.c:412:12: warning: symbol 'omap1_usb1_init' was not declared. Should it be static?
arch/arm/mach-omap1/usb.c:478:12: warning: symbol 'omap1_usb2_init' was not declared. Should it be static?

by declaring those functions as static.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Felipe Balbi <balbi@ti.com>
[tony@atomide.com: this was missed with plat/usb.h removal]
Signed-off-by: Tony Lindgren <tony@atomide.com>