solos: Slight debugging improvements

Print a message if pskb_expand_head fails.

Make atmdebug writable by root, so that you can turn printing of data sent to
and received from the card on and off at runtime - useful for tracking
corruption.

Signed-off-by: Simon Farnsworth <simon@farnz.org.uk>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>