Blackfin: gpio: use shorter name

The gpio label size is 16 char, but the current code uses a longer name
resulting in chopped display.  So use a shorter name.

Reported-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>