ipmi: Fix ioremap error handling in bt-bmc

devm_ioremap_resource returns ERR_PTR so we can't check for NULL.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>