Input: ads7846 - introduce sample settling delay

The ads7846 driver has support for filtering, but when the chip gets
deselected between samples this causes noise. This patch adds support
for an optional settling delay time, so that two consecutive samples
will be taken with the specified delay time apart.  This ensures that
the chip won't be deselected, so the noise won't appear.

Filtering can still be done, but will have less work to do since each
time a new sample is taken the same delay applies.

Signed-off-by: Semih Hazar <semih.hazar@indefia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>