mfd: ti_am335x_tscadc: Remove unwanted reg_se_cache save

In one shot mode, sequencer automatically disables all enabled steps at
the end of each cycle. (both ADC steps and TSC steps) Hence these steps
need not be saved in reg_se_cache for clearing these steps at a later
stage.
Also, when ADC wakes up Sequencer should not be busy executing any of the
config steps except for the charge step. Previously charge step was 1 ADC
clock cycle and hence it was ignored.
TSC steps are always disabled at the end of each conversion cycle, hence
there is no need to explicitly disable TSC steps by writing 0 to REG_SE.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>