drivers/linuxgpiod: release gpio at exit

Commit 20adf85f3484 ("linuxgpiod: add SWDIO buffer") introduces an
additional gpio for SWDIO direction, but does not release it at
driver's exit.

Release the gpio at exit.

Change-Id: If7ea31f79ffed04af585864e49bcf1f35e118bdd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7032
Tested-by: jenkins
Reviewed-by: Steve Marple <stevemarple@googlemail.com>