kgdb,powerpc: Replace hardcoded offset by BREAK_INSTR_SIZE

kgdb_handle_breakpoint checks the first arch_kgdb_breakpoint
which is not known by gdb that's why is necessary jump over
it. The jump lenght is equal to BREAK_INSTR_SIZE that's
why is cleaner to use defined macro instead of hardcoded
non-described offset.
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/kgdb.c

@@ -129,7 +129,7 @@ static int kgdb_handle_breakpoint(struct pt_regs *regs)
 		return 0;
 
 	if (*(u32 *) (regs->nip) == *(u32 *) (&arch_kgdb_ops.gdb_bpt_instr))
-		regs->nip += 4;
+		regs->nip += BREAK_INSTR_SIZE;
 
 	return 1;
 }