staging: rtl8192e: Use macro to represent fragmentation threshold

Add CMDPACKET_FRAG_SIZE macro and use it as fragmentation threshold
in send_cmd_pkt functions.
This makes rt_firmware::cmdpacket_frag_thresold and
 rtl92e_init_fw_param() obsolete.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>