rt2x00: Ignore set_state(STATE_SLEEP) failure

Some hardware never seem to accept the "goto sleep" command, since the legacy
drivers don't have suspend and resume handlers the entire code for it was
basically a educated guess (based on the "enable radio" code).
This patch will only print a warning when the "goto sleep" command fails, and
just continues as usual. Perhaps that means the device will not reach a sleep
state and consumes more power then it should, but it is equally possible it
simply needs some seconds longer to sleep. Anyway, by making the command
non-fatal it will not block the rest of the suspend procedure.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>