WIth the transition from qcserial to USBFS we no longer receive multiple
messages in a single read, so we can remove the code that decode the
buffer piecemeal.

While reworking this, also introduce a mechanism so that the parsers can
pass values back to the caller of the read.

And modify the timeout handling so that the caller always has to pass a
timeout, and bump the timeouts slightly, as they are all fatal anyways.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>