[usb-moded] Fix up some typos and assign another id to connection sharing mode

Some typos could cause some issues for certain modes. The same IdProduct is
assigned to connection sharing as for developer mode as it is basically the
same driver with different network setup.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
Fix up comment about the post synching to avoid confusion in the future.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
[usb-moded] Check return of the dhcp config write

We did not check the return value of the dhcp configuration writing,
which caused mode setting to fail if we did not set up ip forwarding.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>