Input: synaptics-rmi4 - fix register descriptor subpacket map construction

The map_offset variable is specific to the register and needs to be reset
in the loop. Otherwise, subsequent register's subpacket maps will have
their bits set at the wrong index.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Tested-by: Nitin Chaudhary <nitinchaudhary1289@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>