fix usb-hid emulation SET_IDLE

fix usb-hid emulation SET_IDLE

there is a bug in the current usb-hid emulation code that affects the
way the idle flag is handled: the spec clearly states that when SET_IDLE
is called with a value == 0 means "do not send any event unless there is
an actual change". However we are currently assuming the opposite
meaning of the flag.
This patch fixes it and it also fixes a possible initialization issue
that is uncovered by using the default idle flag set to 0.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[supplied patch also trivially changed by Ian Jackson]
(cherry picked from commit 144cb9b13bb57d40b69a5100ab4dd91d9f0036e0)