misc/mei/hdcp: Fix AUTH_STREAM_REQ cmd buffer len

Fix the size of WIRED_REPEATER_AUTH_STREAM_REQ cmd buffer size.
It is based upon the actual number of MST streams and size
of wired_cmd_repeater_auth_stream_req_in.
Excluding the size of hdcp_cmd_header.

v2:
- hdcp_cmd_header size annotation nitpick. [Tomas]

Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Tested-by: Karthik B S <karthik.b.s@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111081120.28417-13-anshuman.gupta@intel.com