x86, ioapic: Simplify probe_nr_irqs_gsi.

Use the global gsi_end value now that all ioapics have
valid gsi numbers instead of a combination of acpi_probe_gsi
and walking all of the ioapics and couting their number of
entries by hand if acpi_probe_gsi gave us an answer we did
not like.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
LKML-Reference: <1269936436-7039-13-git-send-email-ebiederm@xmission.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>