drivers: modem: sara-r4: adjust send and receive lengths

- limit max. number of bytes when sending to socket
  The number of bytes sent to a socket in one transaction
  is limited to 512 in HEX mode (Sara-R4), and to 1024
  otherwise. This corresponds to numbers given in the
  manual for ublox cellular modems.

- report number of bytes actually sent, as reported by modem
  After writing data to a socket, we now return the number of
  bytes actually written, as reported by the modem.

Signed-off-by: Hans Wilmers <hans@wilmers.no>