iommu/arm-smmu: Replace while loop with for_each_set_bit

Switch out a while loop that loops over the set bits in a bitmap with
for_each_set_bit.

Change-Id: I8843a0b36906979a09122a8ccb07c8081278bf3b
Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>