mac80211: Remove redundant checks for NULL before calls to crypto_free_cipher()

crypto_free_cipher() is a wrapper around crypto_free_tfm() which is a
wrapper around crypto_destroy_tfm() and the latter can handle being passed
a NULL pointer, so checking for NULL in the
ieee80211_aes_key_free()/ieee80211_aes_cmac_key_free() wrappers around
crypto_free_cipher() is pointless and just increase object code size
needlesly and makes us execute extra test/branch instructions that we
don't need.
Btw; don't we have to many wrappers around wrappers ad nauseam here?
Anyway, this patch removes the redundant conditionals.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>