check SIM_SCGC1_UART5 in serial6_set_tx

teensy3/serial6.c

 static volatile uint8_t rx_buffer_tail = 0;
 #endif
 
-static uint8_t tx_pin_num = 34;
+static uint8_t tx_pin_num = 48;
 
 // UART0 and UART1 are clocked by F_CPU, UART2 is clocked by F_BUS
 // UART0 has 8 byte fifo, UART1 and UART2 have 1 byte buffer
 
 	if (opendrain) pin |= 128;
 	if (pin == tx_pin_num) return;
-	if ((SIM_SCGC4 & SIM_SCGC4_UART2)) {
+	if ((SIM_SCGC1 |= SIM_SCGC1_UART5)) {
 		switch (tx_pin_num & 127) {
 			case 48:  CORE_PIN48_CONFIG = 0; break; // PTE24
 		}