dma: tegra: fix interrupt name issue with apb dma.

When watching the interrupts through /proc/interrupts,
the name of the interrupts are blank or garbage. The
reason is the pointer passed for devname during irq
registration is stack and so it get changed after
dma registration completes.
Allocate the pointer as part of dma channel and pass
this pointer as the devname for irq registration to
avoid change of name.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reported-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>