Fix wpa_cipher_to_alg() return type

wpa_cipher_to_alg() returns enumerated values from enum wpa_alg and all
uses of the return value treat it as enum wpa_alg (by either assigning
it to a variable of type enum wpa_alg or passing to a function that
expects enum wpa_alg).

This commit updates the return value to match the expected usage
(enum  wpa_alg) rather than int. This ensures the return value is
of the proper type and eliminates the following compiler warnings:

ARM RVCT (2.2):
  'Warning: #188-D: enumerated type mixed with another type'

Signed-off-by: Joel Cunningham <joel.cunningham@me.com>

src/common/wpa_common.c

@@ -1620,7 +1620,7 @@ int wpa_cipher_rsc_len(int cipher)
 }
 
 
-int wpa_cipher_to_alg(int cipher)
+enum wpa_alg wpa_cipher_to_alg(int cipher)
 {
 	switch (cipher) {
 	case WPA_CIPHER_CCMP_256:

src/common/wpa_common.h

@@ -437,7 +437,7 @@ int wpa_ft_parse_ies(const u8 *ies, size_t ies_len, struct wpa_ft_ies *parse);
 
 int wpa_cipher_key_len(int cipher);
 int wpa_cipher_rsc_len(int cipher);
-int wpa_cipher_to_alg(int cipher);
+enum wpa_alg wpa_cipher_to_alg(int cipher);
 int wpa_cipher_valid_group(int cipher);
 int wpa_cipher_valid_pairwise(int cipher);
 int wpa_cipher_valid_mgmt_group(int cipher);