phy: phy-snps-eusb2: fix optional phy lookup parameter

phy: phy-snps-eusb2: fix optional phy lookup parameter

The devm_of_phy_optional_get() takes an optional name argument as its
third parameter and not an index like the recently replaced
devm_of_phy_get_by_index().

Replace 0 with an explicit NULL for consistency and readability.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20250523084839.11015-6-johan+linaro@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>