phy: ti: ensure priv is not null before dereferencing it

Currently priv is being dereferenced before priv is being null checked.
Fix this by moving the null check on priv before the dereference.

Detected by CoverityScan, CID#1476018 ("Dereference before null check")

Fixes: 92b58b34 ("phy: ti: introduce phy-gmii-sel driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>