Use unsigned values for termios setting

This allows the last (most significant) bit to be set without
overflow errors. The error becomes visible after adding the
CRTSCTS constant on linux/386:

   const tcCRTSCTS uint = 0x80000000

serial_darwin_amd64.go

@@ -8,6 +8,6 @@ package serial // import "go.bug.st/serial.v1"
 
 // termios manipulation functions
 
-func termiosMask(data int) uint64 {
+func termiosMask(data uint) uint64 {
 	return uint64(data)
 }