USB: sierra: cleanup the startup and shutdown path

This removes the ugly code that was copied from the keyspan driver and
allocates the in urbs in a much shorter code path that can be understood
easier.

Also turned off the interrupt urb when no port was open as it's not nice
to keep the bus busy for no good reason at all (this should be a power
savings.)

All in all, this saved over 40 lines of code and cleaned things up
better.

Cc: Kevin Lloyd <linux@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>