[Blackfin] arch: Let the pre-processor do the math to save a few cycles - no functional changes

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>

arch/blackfin/mach-common/entry.S

@@ -633,9 +633,7 @@ ENTRY(_ret_from_exception)
 	[sp + PT_IPEND] = r0;
 
 1:
-	r1 = 0x37(Z);
-	r2 = ~r1;
-	r2.h = 0;
+	r2 = LO(~0x37) (Z);
 	r0 = r2 & r0;
 	cc = r0 == 0;
 	if !cc jump 4f;	/* if not return to user mode, get out */