[IA64] move SAL_CACHE_FLUSH check later in boot

The check to see if the firmware drops interrupts during a
SAL_CACHE_FLUSH is done to early in the boot. SAL_CACHE_FLUSH expects
to be able to make PAL calls in virtual mode, on some cell based
machines a fault occurs causing a MCA. This patch moves the check
after mmu_context_init so the TLB and VHPT are properly setup.

Signed-off-by Troy Heber <troy.heber@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>