fix(pycardium): Prevent underflow in sleep function

The loop condition in the epic_sleep based sleep could underflow if an
interrupt introduced too much delay at the end.

pycardium/mphalport.c

@@ -63,7 +63,7 @@ void pycardium_hal_init(void)
 	/*
 	 * Configure SysTick timer for SYSTICK_INTERVAL_US period.
 	 */
-	systick_config(SYSTICK_INTERVAL_US * 32768ULL / 1000000);
+	systick_config(SYSTICK_INTERVAL_US * 32768LL / 1000000);
 }
 
 /******************************************************************************
@@ -157,7 +157,7 @@ void mp_hal_set_interrupt_char(char c)
  * SysTick timer at 1000 Hz
  */
 
-static volatile uint64_t systick_count = 0;
+static volatile int64_t systick_count = 0;
 
 void SysTick_Handler(void)
 {
@@ -167,7 +167,7 @@ void SysTick_Handler(void)
 /*
  * Get an absolute "timestamp" in microseconds.
  */
-static uint64_t systick_get_us()
+static int64_t systick_get_us()
 {
 	uint32_t val, count;
 	uint32_t irqsaved = __get_PRIMASK();
@@ -182,8 +182,8 @@ static uint64_t systick_get_us()
 		__set_PRIMASK(irqsaved);
 	} while (val == 0);
 
-	uint64_t us = count * SYSTICK_INTERVAL_US +
-		      (SysTick->LOAD - val) * 1000000ULL / 32768;
+	int64_t us = count * SYSTICK_INTERVAL_US +
+		     (SysTick->LOAD - val) * 1000000LL / 32768;
 
 	return us;
 }
@@ -244,8 +244,8 @@ static void systick_delay(uint32_t us)
 	if (us < 10000) {
 		systick_delay_precise(us);
 	} else {
-		uint64_t now        = systick_get_us();
-		uint64_t final_time = now + us;
+		int64_t now        = systick_get_us();
+		int64_t final_time = now + us;
 
 		while (final_time - systick_get_us() > 10000) {
 			uint32_t sleep_time =