[Mellanox] support get_transceiver_threshold_info (#3777)

* [sonic_platform.sfp] support get_transceiver_dom_threshold_info_dict

* [platform/sfp]qsfp threshold and beautify code
1. qsfp threshold: tx power
2. beautify code, removing some magic numbers
3. optimize get_present by only reading one byte.

Author: stephenxs