dma: mmp_pdma: fix maximum transfer length

There's no reason for limiting the maximum transfer length to 0x1000.
Take the actual bit mask instead; the PDMA is able to transfer chunks of
up to SZ_8K - 1.

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