drivers: modem_cellular: fix handling of `+C*REG` answers

`+C*REG:` may be received as AT read command answer
or unsolicited notification.
Their syntax differs, and even the overall parameter count varies
depending on what `<n>` is used in the `AT+CEREG=` write command.

To handle all cases properly, check the parameter count and
the presence of the `<tac>` parameter (which is a string and
thus begins with `"`) to figure out what is the position of `<stat>`.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>