iwlwifi: send testmode hcmd reply with rx header

When a host command is sent through testmode, the whole
reply (including rx header) is returned to the user, and not
only the payload of the rx.
Before this commit the length was buggy - the reply contained 4 bytes
after the end of the payload.

Signed-off-by: Amit Beka <amit.beka@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>