USB: Fix unload oops and memory leak in yealink driver

This patch fixes a memory leak and a kernel oops when trying to unload
the driver, due to an unbalanced cleanup.
Thanks Ivar Jensen for spotting my mistake.

Signed-off-by: Henk Vergonet <henk.vergonet@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>