usb: xhci: tegra: Fix error check

In the function tegra_xusb_powerdomain_init(),
dev_pm_domain_attach_by_name() may return NULL in some cases,
so IS_ERR() doesn't meet the requirements. Thus fix it.

Fixes: 6494a9ad

 ("usb: xhci: tegra: Add genpd support")
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20220524121404.18376-1-tangbin@cmss.chinamobile.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>