usb: musb: Fix locking errors for host only mode

If we have USB gadgets disabled and USB_MUSB_HOST set, we get
errors "possible irq lock inverssion dependency detected"
errors during boot.

Let's fix the issue by adding start_musb flag and start
the controller after we're out of the spinlock protected
section.

Reported-by: Ladislav Michl <ladis@linux-mips.org>
Tested-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>