mfd: ti_am335x_tscadc: Update reg_cache variable in clr function

The reg_cache variable should be updated with current
steps in the reg_se register. Then the mask should
apply and clear the register. Previously, the reg_cache
can be an old redundant value that isn't updated.

Signed-off-by: Zubair Lutfullah <zubair.lutfullah@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>