USB: serial: fix hang when opening port

USB: serial: fix hang when opening port

Johan's 'fix use-after-free in TIOCMIWAIT' patchset[1] introduces
one bug which can cause kernel hang when opening port.

This patch initialized the 'port->delta_msr_wait' waitqueue head
to fix the bug which is introduced in 3.9-rc4.

[1], http://marc.info/?l=linux-usb&m=136368139627876&w=2

Cc: stable <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Acked-by: Johan Hovold <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>