USB: gadget: atmel_usba_udc: fix transfer hang issue

When receive data, the RXRDY in status register set by hardware
after a new packet has been stored in the endpoint FIFO. After,
we copy from FIFO, we clear it, make the FIFO can be accessed
again.
In the receive_data() function, this bit RXRDY has been cleared.
So, after the receive_data() function return, this bit should
not be cleared again, or else it will cause the accessing FIFO
corrupt, which will make the data loss.

Signed-off-by: Bo Shen <voice.shen@atmel.com>