misc: mic: fix incorrect use of error codes in SCIF DMA driver

The error code passed to ERR_PTR() always should be negated.  Also, the
return value of scif_add_mmu_notifier() was never checked.

Signed-off-by: Eric Biggers <ebiggers3@gmail.com>
Reviewed-by: Sudeep Dutt <sudeep.dutt@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>