hid: egalax: Report zero as minimum pressure

The firmware reports a logical minimum of one, but in order for
userspace applications to correctly map all reported values to
non-zero pressure, the driver needs to report a logical minimum of
zero.  Fixed with this patch.

Tested-by: Philipp Merkel <mail@philmerk.de>
Cc: Stephane Chatty <chatty@enac.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>