serial: mfd: remove the timeout workaround for A0

This is kind of a revert for commit 669b7a09 "hsu: add a periodic
timer to check dma rx channel", which is a workaround for a bug in A0
stepping silicon, where a dma rx data timeout is missing for some case.
Since new silicon has fixed it and the old version is phasing out, no
need to carry on it any more.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>