drivers: usb_dc_nrfx: Add pre-transfer checks if endpoint is enabled

The shim didn't check if a given endpoint was enabled before requesting
the nrfx_usbd driver to perform a read or write operation on it.
In certain circumstances this led to nrfx_usbd driver being stuck in
a loop waiting for the associated DMA transfer to complete.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>