usb: gadget: dbgp: Fix endpoint config after USB disconnect

SET_FEATURE request with DEBUG_MODE only worked the first time after module
initialisation. Per the USB 2.0 debug device specification, said request
is to be treated as if it were a SET_CONFIGURATION request, i.e. endpoint
must be re-configured.

As configure_endpoints() may now get called multiple times, move it outside
__init and move serial_alloc_tty() call into __init.

Code has assumption that endpoint mapping remains unchanged with consecutive
calls of configure_endpoints().

Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>