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>