arm: dma-mapping: return NULL on remap error

Currently arm_dma_remap returns the offset value when it fails
because of reasons like unavailability of vmalloc space. This
is wrong as the caller expects NULL on failure, and can result
in kernel panic on dereferencing the returned pointer.

Change-Id: Ic851efecd1b5ff1c5e3105f28f392307857e63e3
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>