Technical details:
+Walkera TX sends 8 channels, for MODE 2 TX: right joystick is channel 1 -
+verical, 2 - horizontal,left joystick: 3- vertical, 4- horizontal. Channel 5
+is GERA switch, channel 6 is joystick left, vertical but mixer in TX is
+aplied to it. Channel 7 is available only if TX is configured to send GYRO
+sensitivity (by dip 12 on back side of TX).
+
Driver use interrupt from parport ACK input bit to measure pulse length
using hrtimers.