MIPS: Make check condition for SDBBP consistent with EJTAG spec

According to MIPS EJTAG Specification [1], a Debug Breakpoint
exception occurs when an SDBBP instruction is executed, the
CP0_DEBUG bit DBp indicates that a Debug Breakpoint exception
occurred.

When I read the original code, it looks a little confusing
at first glance, just check bit DBp for SDBBP to make the
code more readable, it will be much easier to understand.

[1] http://www.t-es-t.hu/download/mips/md00047f.pdfSigned-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/mipsregs.h

@@ -1085,6 +1085,10 @@
 #define CVMVMCONF_RMMUSIZEM1_S	0
 #define CVMVMCONF_RMMUSIZEM1	(_U64CAST_(0xff) << CVMVMCONF_RMMUSIZEM1_S)
 
+/* Debug register field definitions */
+#define MIPS_DEBUG_DBP_SHIFT	1
+#define MIPS_DEBUG_DBP		(_ULCAST_(1) << MIPS_DEBUG_DBP_SHIFT)
+
 /*
  * Coprocessor 1 (FPU) register names
  */

arch/mips/kernel/genex.S

@@ -349,8 +349,8 @@ NESTED(ejtag_debug_handler, PT_SIZE, sp)
 	MTC0	k0, CP0_DESAVE
 	mfc0	k0, CP0_DEBUG
 
-	sll	k0, k0, 30	# Check for SDBBP.
-	bgez	k0, ejtag_return
+	andi	k0, k0, MIPS_DEBUG_DBP	# Check for SDBBP.
+	beqz	k0, ejtag_return
 
 #ifdef CONFIG_SMP
 1:	PTR_LA	k0, ejtag_debug_buffer_spinlock