drivers: usb_device: smartbond: Fix reenable of EP0

When EP0 was disabled EP0 was setup to ignore setup and
out packets.
In usb_dc_ep_enable() function mask was never cleared.
It resulted in USB stop functioning after application
disabled endpoint (usb_disable()) and then tried
to enable usb again.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>