staging: wlags49_h2: Make key setting more reliable

Share logic between encodeext and encode, so that we can handle
subtle differences between them (implied set_tx), and clear the
appropriate keys if you attempt to switch straight from WPA to
WEP and vice versa.

Also reinstate the TX buffer flush, and ensure the key index is
written to the card little endian.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>