pmagb-b-fb: improve diagnostics

Add error messages to the probe call.

[adaplas]
On failure, return actual error value instead of -ENXIO.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>