usb: phy: tegra: Fix unnecessary check in tegra_usb_phy_probe()

In the function tegra_usb_phy_probe(), if usb_add_phy_dev() failed,
the return value will be given to err, and if usb_add_phy_dev() succeed,
the return value will be zero. Thus it is unnecessary to repeated check
here.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>