mailbox: PCC: handle parse error

acpi_pcc_probe() calls acpi_table_parse_entries_array() but fails
to check for an error return.  This in turn can result in calling
kcalloc() with a negative count as well as emitting the following
misleading erorr message:

[    2.642015] Could not allocate space for PCC mbox channels

Fixes: 8f8027c5 (mailbox: PCC: erroneous error message when parsing ACPI PCCT)
Signed-off-by: David Arcari <darcari@redhat.com>
Reviewed-by: Al Stone <ahs3@redhat.com>
Cc: 4.18+ <stable@vger.kernel.org> # 4.18+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>