greybus: uart: remove the redundant unregister chrdev

The unregister_chrdev_region() does twice here.
The chrdev region was unregistered
inside tty_unregister_driver().

Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>