dmaengine: PL08x: remove redundant spinlock

The pl08x_driver_data spinlock is only ever initialized.  Nothing else
uses it.  Let's get rid of it.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>