usb: musb: start musb on the udc side, too

I have am335x-evm with one port running in OTG mode. Since commit
fe4cb091 ("usb: musb: gadget: remove hcd initialization") the loaded
gadget does non pop up on the host. All I see is
|usb 4-5: new high-speed USB device number 52 using ehci-pci
|usb 4-5: device descriptor read/64, error -110

Since a later commit 2cc65fea ("usb: musb: add musb_host_setup() and
musb_host_cleanup()) the gadget shows up on the host again but only
in OTG mode (because we have the host init code running). It does not
work in device only mode.
If running in OTG mode and the gadget is removed and added back (rmmod
followed by modprobe of a gadget) then the same error is pops up on the
host side.

This patch ensures that the gadget side also executes musb_start() which
puts the chip in "connect accept" mode. With this change the device
works in OTG & device mode and the gadget can be added & removed
multiple times.
A device (if musb is in OTG mode acting as a host) is only recognized if
it is attached during module load (musb_hdrc module). After the device
unplugged and plugged again the host does not recognize it. We get a
buch of errors if musb running in OTG mode, attached to a host and no
gadget is loaded. Bah.
This is one step forward. Host & device only mode should work. I will
look at OTG later. I looked at this before commit fe4cb091

 and OTG wasn't
working there perfectly so I am not sure that it is a regression :)

Cc: <stable@vger.kernel.org> # v3.11
Cc: Daniel Mack <zonque@gmail.com>
Cc: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>