modem: hl7800: ensure the modem inits with radio on

Ensure the driver enables the LTE radio during init unless boot
in airplane mode is specified.
To ensure the settings applied during init are used, the LTE radio
is turned off during config.
Fix issue where PDP and GPRS connection config could have a mismatched APN.

Signed-off-by: Ryan Erickson <ryan.erickson@lairdconnect.com>