regulator: slg51000: Remove unneeded regl_pdata from struct slg51000

Just use a local variable *ena_gpiod in slg51000_of_parse_cb instead.
With this change, the struct slg51000_pdata can be removed.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>