leds: max8997: Simplify max8997_led_set_mode implementation

It is not necessary to call max8997_update_reg() twice just for updating
MAX8997_REG_LEN_CNTL register. With proper val and mask arguments to
max8997_update_reg() call, this can be done in one max8997_update_reg() call.
And then we can remove max8997_led_clear_mode() function.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>