[Spektrum] Configure the bind pin to be pullup/pulldown.

All Lisa/M and Lisa/MX prior to V2.1 don't have a dedicated pull
resistor on the bind pin. We were configuring the pin as floating input.
This will result in random behaviour, depending on the charge of the
GPIO. The system will decide to set the satellites into bind mode
depending on how much moist finger is present on the pin. We want
something more decisive than that. :)
Merge pull request #814 from esden/spektrum_bind_pin_fix

[Spektrum] Configure the bind pin to be pullup/pulldown.