usb: musb: core: simplify musb_recover_work()

we're not resetting musb at all, just restarting
the session. This means we don't need to touch PHYs
or VBUS or anything like that. Just make sure session
bit is reenabled after MUSB dropped it.

while at that, make sure to tell usbcore that we're
dropping the session and, thus, disconnecting the
device.

Tested-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>