[PATCH] wireless/atmel: fix setting TX key only in ENCODEEXT

The previous patch that added ENCODEEXT and AUTH support to the atmel
driver contained a slight error which would cause just setting the TX
key index to also set the encryption key again.  This patch allows any
combination of setting the TX key index and setting an encryption key.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>