usb: ftdi-elan: remove a unnecessary variable 'empty_packets'

The variable 'empty_packets' does not used in any other places
except for self increment, so it can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>