IB/hfi1: Convert the macro AHG_HEADER_SET into an inline function

AHG_HEADER_SET macro doesn't conform to the coding standards as it can
affect the control flow. Convert the macro AHG_HEADER_SET into an inline
function ahg_header_set().

Cc: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>