config: disable CONFIG_AX88796B_PHY by default

hulk inclusion
category: bugfix
bugzilla: 4979
CVE: NA

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Introduced by f9a8c4166005 ("net: phy: rename Asix Electronics PHY driver")
Disable CONFIG_AX88796B_PHY by default.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>