lan743x: remove redundant assignment to variable rx_process_result

The variable rx_process_result is being initialized with a value that
is never read and is being re-assigned immediately afterwards. The
assignment is redundant, so replace it with the return from function
lan743x_rx_process_packet.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>