drivers/modem sim7080 & ublox-sara: Fix link error

Fix link error when both modems' drivers are enabled
as they both define the same structure but did not
make it static.

This fixes the CI build failure of
tests/drivers/build_all/modem/drivers.modem.simcom_sim7080.build
on particle_boron

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>