The expandable GPIO API requires the comparison of structured GPIO types. This means that inline functions must be used instead of direct comparisons. For the migration process, drivers must first be changed so that they use the inline comparison functions.
In the current implementation, RX is flank based, so it's entirely level
agnostic.
I still chose to include a SOFT_UART_FLAG_INVERT_RX flag for symmetry and
to allow for a possible future, sampling based implementation.
