net: txgbe: initialize PHY interface to 0

DECLARE_PHY_INTERFACE_MASK() does not guarantee zeroed contents. Add a
new macro DECLARE_PHY_INTERFACE_MASK_ZERO(), make the stack variable to
be zeroed before setting supported interfaces.

Fixes: 57d39faed4c9 ("net: txgbe: improve functions of AML 40G devices")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Link: https://patch.msgid.link/20260608070842.36504-4-jiawenwu@trustnetic.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>