swiotlb: remove an unecessary NULL check

Smatch complains here:

    lib/swiotlb.c:730 swiotlb_alloc_buffer()
    warn: variable dereferenced before check 'dev' (see line 716)

"dev" isn't ever NULL in this function so we can just remove the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>