SAMV7: USBHS: Remove disabling of whole usb on suspend

This fix removes the disabling of the whole USB peripheral on suspend
interrupt. Its enough to freeze the clock instead.

When disabling the whole peripheral, the next wakeup-interrupt comes
up with an disabled clocking. The unfreeze clock has no effect, because
the master clock is disabled. This makes all registers, including the
IDR unwriteable and the IRQ falls in an endless loop blocking the whole
system.

Furthermore the disabling of the peripheral clock prevents hotplugging
or reconnecting the USB.