msm_iommu: Check for NULL pointer in msm_detach_dev

The device struct may be passed as NULL into the msm_detach_dev
function. Add check to prevent NULL pointer dereferencing.

Change-Id: I4123b60969358cd4ff9ad20b76257405aacc4257
Signed-off-by: Neeti Desai <neetid@codeaurora.org>