backports: simplify swiotlb_nr_tbl() backport

backports: simplify swiotlb_nr_tbl() backport

swiotlb_nr_tbl() was available since 3.2 but was exported since 3.3.
Since it uses an internal global state variable, it is impossible
to backport it to. Instead of reverting the changes added just
let this return 0 as the code will not do anything. This simplifies
the backport.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>