new-driver: keep driver_info local to the api.c source file

There is a circular reference between the driver info structure (which
lists callback routines) and a driver's routines (one of them may have
to reference the driver info). Add a forward declaration, and keep the
declaration and the implementation 'static' within the api.c source
file. Otherwise linker issues may arise should developers put the
declaration into protocol.h since it's marked as SR_PRIV in the template.