Input: ad7877 - increase pen up imeout

The time interval between consecutive interrupts depends on a number of
tunables: first_conversion_delay, acquisition_time, averaging and foremost
the pen_down_acc_interval.

Since the mod_timer() action for the PEN UP event happens in the
spi_async() callback function, latencies incurred by the spi bus drivers
also need to be taken into account.

So all in all, give the PEN UP event a bit more wiggle room and increase
timeout to 100ms.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>