mp: Correctly mask interrupts

pycardium/mphalport.c

@@ -169,7 +169,7 @@ static int64_t systick_get_us()
 	 * its value can jump to 0 before the interrupt is triggered.
 	 * Simply wait until it is not 0 and then read the count. */
 	do {
-		__set_PRIMASK(0);
+		__set_PRIMASK(1);
 		val   = SysTick->VAL;
 		count = systick_count;
 		__set_PRIMASK(irqsaved);