iwlwifi: handle shared memory

This patch generalize the use of shared memory, as size of this memory is
now allocated and freed by handlers, and also changes the location of those
actions for better resource management

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>