net: phy: ncsi: fixed not nullify the pointers after free

The issue occurs the UAF (use-after-free) to cause double free
when do the realloc function for the pointers during the
reinitialization NC-SI process, and it will cause the memory
management occurs error.
So, nullify these pointers after free.

Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>