iwlwifi: by default, dump entire sram data portion

For "sram" debugfs file, if user did not specify the offset and length,
dump the entire data portion of sram by default.
Data portion is 0x800000 - 0x80ffff, but the actual data size is known
to the driver from the ucode file.

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>