x86/APIC: adjust types and comments in calibrate_APIC_clock()

x86/APIC: adjust types and comments in calibrate_APIC_clock()

First and foremost the comment talking about potential underflow being
taken care of by using signed long type variables was true only on
32-bit, which we've not been supporting for quite some time. Drop the
comment and change all involved types to unsigned. Take the opportunity
and also replace bus_cycle's fixed width type.

Additionally there's no point using an "arbitrary (but long enough)
timeout" here. Just use the maximum possible value; Linux does so too,
just as an additional data point.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>