dma: mmp_pdma: refactor unlocking path in lookup_phy()

As suggested by Ezequiel García, release the spinlock at the end of the
function only, and use a goto for the control flow.

Just a minor cleanup.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>