Fix wpa_cipher_to_alg() return type

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>