input: it7258_ts_i2c: rename regulator_set_optimum_mode function

The function regulator_set_optimum_mode() has been renamed to
regulator_set_load(). So update the driver accordingly.

Change-Id: I7d69a4529368918f159a25769f497e6425838460
Signed-off-by: Abinaya P <abinayap@codeaurora.org>