dma-mapping: Check for valid IOVA address

We might fail in alloc_iova if we run out
of virtual addresses in pool. Currently,
if we fail in alloc_iova we are not checking
for return value and directly going ahead
and mapping invalid IOVA address. Fix this
by explicitly checking for return value
from alloc_iova.

Change-Id: I7681d13b75f47a86f0b0b6cbf4b8405a7528ff66
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>