usb: dwc2: Fix EP TxFIFO number setting

In case when some EP IN is frequently reused, i.e. enabled/disabled by
function driver. It is required to clear TxFIFO number field in DIEPCTL
register before setting new number. Otherwise there is probability to
have same TxFIFO number for different EP's because of OR operator.

Signed-off-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>