libertas: improve reliability of firmware reloading on USB

Increase the delay between issuing the RESET command and the usb reset,
and be prepared to discard more than one 'normal' packet from it before
it resets.

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