mfd: ti_am335x_tscadc: Make am335x_tsc_se_update() local

Since the "recent" changes, am335x_tsc_se_update() has no longer any
users outside of this file so make it local.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>