regmap: fix NULL pointer dereference in regmap_get_val_endian

Recents commits for getting reg endianness causing NULL pointer
dereference if dev is passed NULL in regmap_init_mmio. This patch
fixes this issue, and allows to parse reg endianness only if dev
and dev->of_node exist.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>