xtensa: fix initialization of pt_regs::syscall in start_thread

New pt_regs should indicate that there's no syscall, not that there's
syscall #0. While at it wrap macro body in do/while and parenthesize
macro arguments.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/processor.h

@@ -187,15 +187,18 @@ struct thread_struct {
 
 /* Clearing a0 terminates the backtrace. */
 #define start_thread(regs, new_pc, new_sp) \
-	memset(regs, 0, sizeof(*regs)); \
-	regs->pc = new_pc; \
-	regs->ps = USER_PS_VALUE; \
-	regs->areg[1] = new_sp; \
-	regs->areg[0] = 0; \
-	regs->wmask = 1; \
-	regs->depc = 0; \
-	regs->windowbase = 0; \
-	regs->windowstart = 1;
+	do { \
+		memset((regs), 0, sizeof(*(regs))); \
+		(regs)->pc = (new_pc); \
+		(regs)->ps = USER_PS_VALUE; \
+		(regs)->areg[1] = (new_sp); \
+		(regs)->areg[0] = 0; \
+		(regs)->wmask = 1; \
+		(regs)->depc = 0; \
+		(regs)->windowbase = 0; \
+		(regs)->windowstart = 1; \
+		(regs)->syscall = NO_SYSCALL; \
+	} while (0)
 
 /* Forward declaration */
 struct task_struct;