pata_at91: fix ata_host_activate() failure handling

Add missing clk_put() call to ata_host_activate() failure path.

Sergei says,

  "Hm, I have once fixed that (see that *if* (!ret)) but looks like a
   later commit 477c87e9 (ARM:
   at91/pata: use gpio_is_valid to check the gpio) broke it again. :-(
   Would be good if the changelog did mention that..."

Cc: Andrew Victor <linux@maxim.org.za>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org