swiotlb: don't override user specified size in swiotlb_adjust_size

If the user already specified a swiotlb size on the command line,
swiotlb_adjust_size should not overwrite it.

Fixes: 2cbc2776

 ("swiotlb: remove swiotlb_nr_tbl")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>