Don't overwrite SWP_CFG on SE pipe enable/disable.

Different BCM2079x revisions have different interpretations for
bits in the SWP_CFG config parameter. Instead of overwriting
whatever is there, read the current value, and just set/unset
the bit for UICC_IDLE timeout as desired.

Bug: 8776949
Change-Id: I33807aef1e94cfa6a2f4af0dd6a46dae630d850f