greybus: properly drop device reference

Drop the USB device reference taken at the top of ap_probe() in the
event greybus_create_hd() fails.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
greybus: create a slab cache for operations

Everything we do on greybus will involve an operation, so create a
slab cache for that frequently-allocated data structure.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>