p2sb: Introduce the global flag p2sb_hidden_by_bios

To prepare for the following fix, introduce the global flag
p2sb_hidden_by_bios. Check if the BIOS hides the P2SB device and store
the result in the flag. This allows to refer to the check result across
functions.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20241128002836.373745-3-shinichiro.kawasaki@wdc.com


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>