[802.11] Use correct SHA1_DIGEST_SIZE constant name

The constant SHA1_SIZE is defined only as part of the imported AXTLS code.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/80211/wpa_ccmp.c

 {
 	u8 sha1_ctx[SHA1_CTX_SIZE];
 	u8 kckb[16];
-	u8 hash[SHA1_SIZE];
+	u8 hash[SHA1_DIGEST_SIZE];
 	size_t kck_len = 16;
 
 	memcpy ( kckb, kck, kck_len );