uart: skip frames with invalid start bits

uart: skip frames with invalid start bits

When the start bit is not low at its sample point, then stop trying
to interpret the remaining frame -- it's already known to be invalid,
anyway.

Wait for the next start bit instead, assuming that either the falling
edge which started the inspection of the UART frame and its start bit
was a spurious glitch or that the captured signal does not communicate
at the decoder's configured bitrate.

Signed-off-by: Gerhard Sittig <redacted>