[IA64] Split 16-bit severity field in sal_log_record_header

ERR_SEVERITY item is defined as a 8 bits item in SAL documentation
($B.2.1 rev december 2003), but as an u16 in sal.h.
This has the side effect that current code in mca.c may not call
ia64_sal_clear_state_info() upon receiving corrected platform errors
if there are bits set in the validation byte.  Reported by Xavier Bru.

Signed-off-by: Tony Luck <tony.luck@intel.com>