regulator: lp8788-buck: Remove lp8788_set_default_dvs_ctrl_mode function

We already know the mask in lp8788_init_dvs() function, and we can update
the corresponding bit for default_dvs_mode in lp8788_init_dvs() function.
This function looks not necessary to me.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Tested-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>