x86: use car_(get|set)_var accessors for apic timer

The timer_fsb variable was not correctly being accessed in the
presence of cache-as-ram. The cache-as-ram backing store could
be torn down but then udelay() could be called causing hangs from
accessing variables that have unknown values.

Instead change the timer_fsb variable to g_timer_fsb and obtain
the value through a local access method that does the correct things
to obtain the correct value.

Change-Id: Ia3e30808498cbe4a7f6f116c17a8cf1240a807a3
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5411
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)