mwifiex: fix skb length issue when send a command to firmware

When we send a command to firmware, we assumed that cmd_size
will be always less than or equal to the structure size of
host_cmd_ds_command. However, this is no longer true after
we added AP support. There are some AP commands that Custom
IE TLVs are included in command buffer, hence the cmd_size
gets enlarged by the TLV data. We need to increase the skb
length for the extra data.

Signed-off-by: Stone Piao <piaoyun@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>