mwifiex: memory corruption in mwifiex_tdls_add_vht_capab()

There is a typo here because the names are confusingly similar.  The
intent was sizeof(struct ieee80211_vht_cap) (size 12) but sizeof(struct
ieee80211_ht_cap) (size 32) was used.

Anway, it's cleaner to just specify the variable instead of the type.

Fixes: 5f6d5983 ('mwifiex: add VHT support for TDLS')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>