iwlagn: modify the debug macro to be usable by all the layers

Since all the layers need to print debug message, the debug macro cannot suppose
that they will be given iwl_priv as a parameter and then dereference it.
Use iwl_shared instead.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>