gril: Print slot in traces
ril: Print slot in trace for completeness
rilmodem: Add modem to gprs driver data

The modem pointer is needed for callbacks to mtk plugin.
mtkmodem: Inform mtk plugin of atachment state

mtk plugin needs this information to act in the other modem of a
dual SIM set-up.
mtk: Fix issue with data attachment

When org.ofono.ConnectionManager Powered property was set for, say,
slot 1 while slot 0 had that property also set, slot 1 was not
attaching even after slot 0 data connection was finally dropped.
This commit fixes that problem.