Revert "Modem: rename config option to CONFIG_308_MODEM"

This reverts commit 978653495eb01300ab4416520e4f953eaa2e2a06.
Revert "Edison: modem: Rename config option to CONFIG_BACKPORT_MODEM"

This reverts commit 41375cbad2a46b5931c8be9cd41c5a60afff95fb.