mwifiex: use 'const' qualifier for 2nd arg of mwifiex_tdls_add_ht_oper

Fixes the following warning:

  CC      drivers/net/wireless/mwifiex/tdls.o
drivers/net/wireless/mwifiex/tdls.c: In function ‘mwifiex_prep_tdls_encap_data’:
drivers/net/wireless/mwifiex/tdls.c:475:4: warning: passing argument 2 of ‘mwifiex_tdls_add_ht_oper’ discards ‘const’ qualifier from pointer target type [enabled by
default]
    ret = mwifiex_tdls_add_ht_oper(priv, peer, 1, skb);
    ^
drivers/net/wireless/mwifiex/tdls.c:190:1: note: expected ‘u8 *’ but argument is of type ‘const u8 *’
 mwifiex_tdls_add_ht_oper(struct mwifiex_private *priv, u8 *mac,
 ^
drivers/net/wireless/mwifiex/tdls.c:481:4: warning: passing argument 2 of ‘mwifiex_tdls_add_ht_oper’ discards ‘const’ qualifier from pointer target type [enabled by
default]
    ret = mwifiex_tdls_add_ht_oper(priv, peer, 0, skb);
    ^
drivers/net/wireless/mwifiex/tdls.c:190:1: note: expected ‘u8 *’ but argument is of type ‘const u8 *’
 mwifiex_tdls_add_ht_oper(struct mwifiex_private *priv, u8 *mac,
 ^

Signed-off-by: John W. Linville <linville@tuxdriver.com>