libertas: discard DEFER responses to commands; let the timeout trigger

When the firmware returns 0x0004, it wants us to try again later. We can
achieve that simply by throwing out the response and letting the command
timeout code kick in.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>