staging: rtl8192u: ieee80211: remove set but not used variable 'data_len'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/staging/rtl8192u/ieee80211/ieee80211_crypt_ccmp.c:
 In function ieee80211_ccmp_encrypt:
drivers/staging/rtl8192u/ieee80211/ieee80211_crypt_ccmp.c:162:6:
 warning: variable data_len set but not used [-Wunused-but-set-variable]

It is not used since commit eb0e7bf3 ("staging:
rtl8192u: ieee80211: ieee80211_crypt_ccmp.c: Use crypto API ccm(aes)")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20190821122250.71404-1-yuehaibing@huawei.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8192u/ieee80211/ieee80211_crypt_ccmp.c

@@ -159,7 +159,7 @@ static int ccmp_init_iv_and_aad(struct rtl_80211_hdr_4addr *hdr,
 static int ieee80211_ccmp_encrypt(struct sk_buff *skb, int hdr_len, void *priv)
 {
 	struct ieee80211_ccmp_data *key = priv;
-	int data_len, i;
+	int i;
 	u8 *pos;
 	struct rtl_80211_hdr_4addr *hdr;
 	struct cb_desc *tcb_desc = (struct cb_desc *)(skb->cb + MAX_DEV_ADDR_SIZE);
@@ -169,7 +169,6 @@ static int ieee80211_ccmp_encrypt(struct sk_buff *skb, int hdr_len, void *priv)
 	    skb->len < hdr_len)
 		return -1;
 
-	data_len = skb->len - hdr_len;
 	pos = skb_push(skb, CCMP_HDR_LEN);
 	memmove(pos, pos + CCMP_HDR_LEN, hdr_len);
 	pos += hdr_len;