phy: Set phy->dev to NULL when generic_phy_get_by_name() fails

generic_phy_get_by_name() does not initialize phy->dev to NULL before
returning when dev_read_stringlist_search() fails. This can lead to an
uninitialized or reused struct phy erroneously be report as valid by
generic_phy_valid().

Fix this issue by initializing phy->dev to NULL, also extend the
dm_test_phy_base test with calls to generic_phy_valid().

Fixes: b9688df3 ("drivers: phy: Set phy->dev to NULL when generic_phy_get_by_index() fails")
Fixes: 868d58f6 ("usb: dwc3: Fix non-usb3 configurations")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>