ARM: OMAP: fix section mismatches in usb-host.c

Fix the below section mismatch warning and alike:

WARNING: vmlinux.o(.text+0x281d4): Section mismatch in reference from
the function setup_ehci_io_mux() to the function
.init.text:omap_mux_init_signal()
The function setup_ehci_io_mux() references
the function __init omap_mux_init_signal().
This is often because setup_ehci_io_mux lacks a __init
annotation or the annotation of omap_mux_init_signal is wrong.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Reviewed-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>