phy: phy-mvebu-sata: Add missing error check for devm_kzalloc

Currently this driver is missing a check on the return value of devm_kzalloc,
which would cause a NULL pointer dereference in a OOM situation.

This patch adds the aformentioned missing check.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>