drivers: modem: gsm_ppp: hold off ppp until we're attached to nw

Attempting to initialize PPP without first ensuring we're attached to
the network packet service will sometimes lead to "NO CARRIER" errors
when we initialize PPP with the modem. This has been observed
reproducibly on some SIMCOM7600E modems.

This commit holds off on PPP initialization until modem has reported
that it is indeed attached by using the "AT+CGATT?" command (see 3GPP TS
27.007)

Signed-off-by: Benjamin Lindqvist <benjamin.lindqvist@endian.se>