platform/x86: p2sb: Make p2sb_get_devfn() return void

p2sb_get_devfn() always succeeds, make it return void and
remove error checking from its callers.

Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240305094500.23778-1-hdegoede@redhat.com