ACPI: TAD: Rearrange RT data validation checking

ACPI: TAD: Rearrange RT data validation checking

Move RT data validation checks from acpi_tad_set_real_time() to
a separate function called acpi_tad_rt_is_invalid() and use it
also in acpi_tad_get_real_time() to validate data coming from
the platform firmware.

Also make acpi_tad_set_real_time() return -EINVAL when the RT data
passed to it is invalid (instead of -ERANGE which is somewhat
confusing) and introduce ACPI_TAD_TZ_UNSPEC to represent the
"unspecified timezone" value.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/3409319.aeNJFYEL58@rafael.j.wysocki