ARM: 7237/1: PL330: Fix driver freeze

Add a req_running field to the pl330_thread to track which request (if
any) has been submitted to the DMA.  This mechanism replaces the old
one in which we tried to guess the same by looking at the PC of the
DMA, which could prevent the driver from sending more requests if it
didn't guess correctly.

Reference: <1323631637-9610-1-git-send-email-javi.merino@arm.com>

Signed-off-by: Javi Merino <javi.merino@arm.com>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Tested-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>