When sysfs is simultaneously changing while psud updating, some data could be outdated immediately once read, so delay the psu presence and status updating to the final step, insure we get the latest info.

Description
PSU Model Serial HW Rev Voltage (V) Current (A) Power (W) Status LED
PSU 1 N/A N/A N/A 12.00 N/A N/A OK green
PSU 2 XXXXXXX XXXXXXXX A3 12.01 16.25 194.75 OK green

The status should be NOTOK as the psu had been turned off and other fields had became N/A as expected.

Motivation and Context
The failure flow are showing below:

    psu_db_update(self.psu_tbl, self.num_psus) // here we read and update the old data of psu status. 
    self.update_psu_data() // now psu status had been changed to NOTOK, but we won’t read it until next psu update cycle , other filed becomes N/A as expected.
How Has This Been Tested?
platform_tests/api/test_psu.py

Signed-off-by: Yuanzhe, Liu <yualiu@nvidia.com>