defxx: Fix the handling of ioremap() failures

 If ioremap_nocache() is unfortunate enough to fail, the error code is not
set correctly leading to a false success from dfx_register().  This change
fixes the problem.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>