mac80111: aes_gcm: clean up ieee80211_aes_gcm_key_setup_encrypt()

This code is written using an anti-pattern called "success handling"
which makes it hard to read, especially if you are used to normal kernel
style.  It should instead be written as a list of directives in a row
with branches for error handling.

(Basically copied from Dan's previous patch for CCM)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>