mwifiex: do not send key material cmd when delete wep key

This patch fixes memory corruption reported by community developer.

"Memory corruption occurs in mwifiex_ret_802_11_key_material_v1()
when a short command response is received without a key length
causing non initialised memory to be interpreted as the key
length resulting in a memcpy() overwriting the part of the
driver's private data structure beyond the key area."

For v1 key material API firmwares, there is no need to send
command to delete WEP key. WEP encryption/decryption is controlled
by mac_control command.

This patch avoids sending key material command in del_key case.

Reported-by: Martin Fuzzey <mfuzzey@parkeon.com>
Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>