USB: udc: atmel_usba_udc: no need to check return value of debugfs_create functions

When calling debugfs functions, there is no need to ever check the
return value.  The function can work or not, but the code logic should
never do something different based on this.

There is also no need to keep the file dentries around at all, so remove
those variables from the device structure.

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Felipe Balbi <balbi@kernel.org>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>