Construction of a MAC with decryption mode of a block cipher
A block cipher is a (strong) pseudorandom permutation. As such, it makes no difference if you compute it in the forward or in the backward direction. Thus, you can use AES in decryption mode and it has exactly the same security as AES in encryption mode, as a block cipher. Therefore, you can do CBC-MAC with AES decryption and you are fine.

Two remarks:


Make sure you use an independent key for the CBC-MAC (and not the same as the encryption), and
You should use CMAC and not straight CBC-MAC.