superio: Use 'pnp_devfn_t' over 'device_t' in romstage component

The romstage component of Super I/O support is in fact written around
passing a lower and upper half packed integer. We currently have two
typedef's for this, 'device_t' and 'pnp_devfn_t'. We wish to make use of
'pnp_devfn_t' over 'device_t' as 'device_t' changes it's typedef in the
ramstage context and so is really a conflicting definition. This helps
solve problems down the road to having the 'real' 'device_t' definition
usable in romstage later.

Change-Id: Ia9f238ebb944f9fe7b274621ee0c09a6de288a76
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>