ACPI / APEI: Fix crash in apei_hest_parse() for acpi=off

After commit 92ef2a2 (ACPI: Change the ordering of PCI root bridge
driver registrarion), acpi_hest_init() is never called for acpi=off
(acpi_disabled), so hest_disable is not set, but hest_tab is NULL,
which causes apei_hest_parse() to crash when it is called from
aer_acpi_firmware_first().

Fix that by making apei_hest_parse() check if hest_tab is not NULL
in addition to checking hest_disable.  Also remove the now useless
acpi_disabled check from apei_hest_parse().

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>