norma-dmm: Remove unused version field from state struct

norma-dmm: Remove unused version field from state struct

The version field in the norma-dmm driver state struct is initialized, but
never read and the same information is also already available in the
sr_dev_inst struct version field. So remove it to simplify the code.

This allows to get rid of the custom dev_close() callback and just use the
standard std_serial_dev_close() instead.

Signed-off-by: Lars-Peter Clausen <redacted>