ata: pata_legacy: make legacy_exit() work again

Commit defc9cd8 ("pata_legacy: resychronize with upstream changes and
resubmit") missed to update legacy_exit(), so that it now fails to do any
cleanup -- the loop body there can never be entered.  Fix that and finally
remove now useless nr_legacy_host variable...

Found by Linux Verification Center (linuxtesting.org) with the Svace static
analysis tool.

Fixes: defc9cd8

 ("pata_legacy: resychronize with upstream changes and resubmit")
Cc: stable@vger.kernel.org
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>