backports: remove swiotlb_nr_tbl()

Upstream commit 582829f7d8c136730ee9f5e1d90d4d157b93602b

This is not used anywhere.
The Debian Wheezy kernel also includes this function and it conflicted
with backports version.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>