usb: gadget: f_accessory: Fix check for dev->disconnected

The previous commit d378aeae512d ("usb: gadget: f_accessory: avoid
requeuing write req if disconnected") added a check for
dev->disconnected during write operation, but incorrectly
inverted the value, thus it fails when connected. Fix the polarity
of this check.

Change-Id: I447871d054e35a9d21eb9e55087facfbf7701dd5
Signed-off-by: Jack Pham <jackp@codeaurora.org>