staging: et131x: Reduce indenting in et131x_rx_dma_memory_free()

This change negates an 'if' statement, allowing a large block of code
to be un-indented, making the code more readable.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>