Modifying the macro name Main_Ref to all caps(MAIN_REF).

Suggested by Nishanth Menon <nm@ti.com>
Signed-off-by: NLesly A M <leslyam@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: David Derrick <dderrick@ti.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>