usb: chipidea: udc: rework ep_enable cap setting

This patch reworks the cap value from several read
and write operations to one single operation.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Reviewed-by: Felipe Balbi <balbi@ti.com>
[Alex: removed useless isoc-related bit of code]
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>