serial_posix: fix even parity

Previously, when even parity was requested, PARODD was erroneously retained, and thus could actually result in odd parity being set (retained from what was set by tcgetattr)

serial_posix.go

@@ -113,6 +113,7 @@ func openPort(name string, baud int, databits byte, parity Parity, stopbits Stop
 		st.c_cflag |= C.PARODD
 	case ParityEven:
 		st.c_cflag |= C.PARENB
+		st.c_cflag &= ^C.tcflag_t(C.PARODD)
 	default:
 		return nil, ErrBadParity
 	}