drivers: serial: nrfx_uart: Fix s32_t usage

s32_t was used instead of int32_t after the type transition in Zephyr.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

drivers/serial/uart_nrfx_uart.c

@@ -82,7 +82,7 @@ static struct {
 	size_t tx_buffer_length;
 	volatile size_t tx_counter;
 #if HW_FLOW_CONTROL_AVAILABLE
-	s32_t tx_timeout;
+	int32_t tx_timeout;
 	struct k_delayed_work tx_timeout_work;
 #endif
 } uart0_cb;