Fix poor accuracy due to repeat open and close of NIC.

[sueastward]

use case

ec_init
ec_config_init //timeBeginPeriod(1) call once
//do something
close_nic //timeEndPeriod(1)
//continue
ec_init
ec_config_init // timeBeginPeriod(1) will not called
//poor accuracy
close_nic //timeEndPeriod(1)

This fix resolves this issue.

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