Input: xilinx_ps2 - allocate serio port separately

'struct serio' is a refcounted data structure with lifetime rules different
from 'struct xps2data'. It is quite likely that serio_unregister_port() will
try to free memory allocated by the port and that is why it should be
allocated separately.

Also switch to using platform_get/set_drvdata instead of dev_get/set_drvdata
because we are dealing with platform device.

Reported-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>