Input: synaptics-rmi4 - use the RMI_F11_REL_BYTES define in rmi_f11_rel_pos_report

The size of relative data in F11 is already defined by RMI_F11_REL_BYTES.
Use the define in rmi_f11_rel_pos_report() to be consistent.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Reviewed-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>