iwlwifi: add generic chunk size of tm dumpit packet

Use generic chunk size of dumpit packet for
all necessary testmode commands instead of
add declaration individually.

Signed-off-by: Kenny Hsu <kenny.hsu@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>