drivers: modem: Fix PDP context management for BG9X

The PDP context might be active.
If that's the case, the AT+QIACT command returns an error.
It's then not possible to succeed at setting up the module.

Furthermore, we apply the logic described in the Quectel Documentation :
- If there is an issue 3 consecutive times on activating/deactivating
the context, we restart the module.
- If the AT+QIDEAT command returns an error, we restart the module.

This PR is bug-fix aimed.
We leave parameterization of context ID for future enhancement.

Signed-off-by: Thomas LE ROUX <thomas.leroux@smile.fr>