ARM: davinci: edma: fix dmaengine induced null pointer dereference on da830

This adds additional error checking to the private edma api implementation
to catch the case where the edma_alloc_slot() has an invalid controller
parameter. The edma dmaengine wrapper driver relies on this condition
being handled in order to avoid setting up a second edma dmaengine
instance on DA830.

Verfied using a DA850 with the second EDMA controller platform instance
removed to simulate a DA830 which only has a single EDMA controller.

Reported-by: Tomas Novotny <tomas@novotny.cz>
Signed-off-by: Matt Porter <mporter@ti.com>
Cc: stable@vger.kernel.org # v3.7.x+
Tested-by: Tomas Novotny <tomas@novotny.cz>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>