greybus: uart: Fix minor number leak

On the gb_uart_remove() path we are forgetting to do a release_minor()
leading to a minor number leak. This is a simple one-line fix.

Tested on EVT 2.0

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>