net: usb: lan78xx: scan all MDIO addresses on LAN7801

The LAN7801 is designed exclusively for external PHYs (unlike the
LAN7800/LAN7850 which have internal PHYs), but lan78xx_mdio_init()
restricts PHY scanning to MDIO addresses 0-7 by setting phy_mask to
~(0xFF). This prevents discovery of external PHYs wired to addresses
outside that range.

One such case is the DP83TC814 100BASE-T1 PHY, which is typically
configured at MDIO address 10 via PHYAD bootstrap pins and goes
undetected with the current mask.

Remove the restrictive phy_mask assignment for the LAN7801 so that the
default mask of 0 applies, allowing all 32 MDIO addresses to be
scanned during bus registration.

Fixes: 02dc1f3d613d ("lan78xx: add LAN7801 MAC only support")
Signed-off-by: Martin Pålsson <martin@poleshift.se>
Link: https://patch.msgid.link/0110019c6f388aff-98d99cf0-4425-4fff-b16b-dea5ad8fafe0-000000@eu-north-1.amazonses.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>