staging: wlan-ng: improved readability of function prism2_add_key

Improve readability of prism2_add_key:
a) Reduce nesting and removed goto statement by using more return statements.

Signed-off-by: Chris Opperman <eklikeroomys@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>