This new error code 'MM_SERIAL_ERROR_SEND_TIMEOUT' is used to make it easier
to distinguish why the transmission failed. Until now,
'MM_SERIAL_ERROR_SEND_FAILED' was always returned. This new error code is now
returned for an 'EAGAIN', as this is more likely to be a TIMEOUT.
This makes debugging easier, as a better error analysis is now possible.
Signed-off-by: Florian Eckert <fe@dev.tdt.de>