usb: phy: mxs: do not set PWD.RXPWD1PT1 for low speed connection

At very rare cases, the SoF will not send out after resume with
low speed connection. The workaround is do not power down
PWD.RXPWD1PT1 bit during the suspend.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>