usb: musb: am35x: fix error return code in am35x_probe()

Fix to return a negative error code from the usb_phy_generic_register()
error handling case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>