mwifiex: update command response skb length correctly

Same skb is being reused for storing command response from firmware
in PCIe chipsets. There was a bug while updating the skb length.
This patch ensures skb length correctly gets updated based on rx_len.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>