modemmanager: fix custom initial EPS bearer username

modemmanager: fix custom initial EPS bearer username

The custom initial EPS bearer code uses init_username, which agrees
with the documented UCI option. However, the option was registered
and read as init_user, so init_username was discarded before the
custom EPS bearer settings were passed to mmcli.

Register and read the option consistently as init_username.

Signed-off-by: Michael Pfeifroth <redacted>