linux_driver: change when the buffers are marked as valid

This commit initializes RX buffers as valid at creation and when RX is
(re)enabled.
libbladeRF: change to using BLADERF_GPIO_FEATURE_HW macro

Minor clean up.
libbladeRF: Fixed typos introduced in previous commit

The patch applied as f6018c14d9e8b09b22b3d1be9eb8ee9e84818396 wasn't
intended to be merged. This cleans up said patch and touches
up surrounding documentation a bit.

This compiles but is untested as I don't currently have access to
hardware. (Just wanted to ensure the build isn't left broken all day!)

My apologies if this inconvenience anyone!