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_386.go

@@ -6,6 +6,6 @@
 
 package serial // import "go.bug.st/serial.v1"
 
-func termiosMask(data int) uint32 {
+func termiosMask(data uint) uint32 {
 	return uint32(data)
 }