powerpc/fsl-rio: Fix fsl_rio_setup error paths and use-after-unmap

Several of the error paths from fsl_rio_setup are missing error
messages.

Worse, fsl_rio_setup initializes several global pointers and does not
NULL them out after freeing/unmapping on error.  This caused
fsl_rio_mcheck_exception() to crash when accessing rio_regs_win which
was non-NULL but had been unmapped.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Liu Gang <Gang.Liu@freescale.com>
---
Liu Gang, are you sure all of these error conditions are fatal?  Why
does the rio driver fail if rmu is not present (e.g.  on t4240)?