gianfar: Fix thinko in gfar_set_rx_stash_index()

We obviously want to write a modified 'temp' value back to the
register, not the saved IRQ flags.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>