Staging: iio: fix duplicate dev_attr_name

device attr's should be static, otherwise duplicate identifiers are
created:

drivers/staging/iio/trigger/iio-trig-gpio.o:(.data+0x1c): multiple definition of `dev_attr_name'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>