[rilmodem] Do not remove unexisting modem data

If ril connection fails when called from reattempt, timer interval
should not be reset. In that case method called by timer does not
return false and that leads to double timer. If modem data does
not exist in the core it should not be removed.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #200 from jkangas/master

  Request for pulling prevention of modem data reseting if modem is not registered to master branch
[gprs] Limit MMS interface MTU by 1280

This improves interoperability with certain operators, notably Saunalahti.
The limit of 1280 was chosen because it's the minimum MTU required from
IPv6 capable networks.
Merge pull request #202 from monich/mms-pdu

[gprs] Limit MMS interface MTU by 1280