net: netcp: add error check to netcp_allocate_rx_buf()

Currently, if netcp_allocate_rx_buf() fails due no descriptors
in the rx free descriptor queue, inside the netcp_rxpool_refill() function
the iterative loop to fill buffers doesn't terminate right away. So modify
the netcp_allocate_rx_buf() to return an error code and use it break the
loop when there is error.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>