serial: sh-sci: Fix up ioremap handling.

We were using an IS_ERR() check for the ioremap case, presumably because
this matched the old custom ioremap call that sh64 was providing. Now
that all ioremap() implementations trap the IS_ERR case and hand back a
NULL, check for that instead.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>