pata_arasan_cf: declare/use more local variables in arasan_cf_dma_start()

'acdev->qc', 'acdev->qc->ap', and 'acdev->qc->tf' expressions are used multiple
times in this function, so it makes sense to use the local variables for them.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>