usb: musb: fix dependency on transceiver driver

usb: musb: fix dependency on transceiver driver

This patch let glue driver return -EPROBE_DEFER if the transceiver
is not readly, so we can support defer probe on musb to fix the
below error on 3.7-rc5 if transceiver drivers are built as module:

[   19.052490] unable to find transceiver of type USB2 PHY
[   19.072052] HS USB OTG: no transceiver configured
[   19.076995] musb-hdrc musb-hdrc.0.auto: musb_init_controller failed with status -19
[   19.089355] musb-hdrc: probe of musb-hdrc.0.auto rejects match -19
[   19.096771] driver: 'musb-omap2430': driver_bound: bound to device 'musb-omap2430'
[   19.105194] bus: 'platform': really_probe: bound device musb-omap2430 to driver musb-omap2430
[   19.174407] bus: 'platform': add driver twl4030_usb
[   19.179656] bus: 'platform': driver_probe_device: matched device twl4030_usb with driver twl4030_usb
[   19.202270] bus: 'platform': really_probe: probing driver twl4030_usb with device twl4030_usb
[   19.214172] twl4030_usb twl4030_usb: HW_CONDITIONS 0xc0/192; link 3
[   19.239624] musb-omap2430 musb-omap2430: musb core is not yet ready
[   19.246765] twl4030_usb twl4030_usb: Initialized TWL4030 USB module
[   19.254516] driver: 'twl4030_usb': driver_bound: bound to device 'twl4030_usb'
[   19.263580] bus: 'platform': really_probe: bound device twl4030_usb to driver twl4030_usb

Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: <stable@vger.kernel.org> v3.8
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>