firewire: nosy: do not ignore errors in ioremap_nocache()

There is no check if ioremap_nocache() returns a valid pointer.
Potentially it can lead to null pointer dereference.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (renamed goto labels)