usb: phy: mxs: workaround timeout

This works around that issue:

[    1.127533] stmp_reset_block(c8828030): module reset timeout
[    1.133240] imx_usb 80090000.usb: unable to init phy: -110
[    1.138982] imx_usb: probe of 80090000.usb failed with error -110

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>