iwlwifi: dynamically allocate buffer for sram debugfs file

Dynamically allocate memory for dumping SRAM based on the length of memory
to be displayed.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>