[Mellanox] Fix wrong type of parameter in mlnx-sfpd notification

[keboliu]

- What I did
fix an mlnx-sfpd bug that sends out notifications with the wrong type of parameters:

May 28 06:39:23.777382 arc-mtbc-1001 INFO syncd#mlnx-sfpd: SFP on port 48 state 0
May 28 06:39:23.779545 arc-mtbc-1001 INFO pmon#supervisord: xcvrd terminate called after throwing an instance of 'std::domain_error'
May 28 06:39:23.779856 arc-mtbc-1001 INFO pmon#supervisord: xcvrd   what():  type must be string, but is number
May 28 06:39:23.827453 arc-mtbc-1001 ERR syncd#mlnx-sfpd: unknown module state 0 on port 48
May 28 06:39:26.849262 arc-mtbc-1001 INFO syncd#mlnx-sfpd: SFP on port 48 state 

- How I did it
change the type of the parameter to string.

- How to verify it
trigger cable plug in/plug out event on Mellanox platform and see the result of xcvrd.
with this fix, events sends to xcvrd with expected parameter type and can be handled correctly.

May 28 06:31:32.561216 arc-mtbc-1001 INFO syncd#mlnx-sfpd: SFP on port 48 state 0
May 28 06:31:32.561921 arc-mtbc-1001 INFO pmon#xcvrd: Got SFP removed event
May 28 06:31:32.563551 arc-mtbc-1001 ERR syncd#mlnx-sfpd: unknown module state 0 on port 48
May 28 06:31:35.635751 arc-mtbc-1001 INFO syncd#mlnx-sfpd: SFP on port 48 state 1

- Description for the changelog

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