Input: synaptics-rmi4 - fix maximum size check for F12 control register 8

According to the RMI4 spec the maximum size of F12 control register 8 is
15 bytes. The current code incorrectly reports an error if control 8 is
greater then 14. Making sensors with a control register 8 with 15 bytes
unusable.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Reported-by: Chris Healy <cphealy@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>