drivers: sensor: st: qdec_stm32: addition of fractional part

The fractional part of the qdec sensor readout is set to zero (val2 =
0).

Changed the format of the run time data to Q26.6 and adjusted the
assignment of val1 and val2 accordingly.

Signed-off-by: Juraj Lieskovský <lieskovsky.juraj@gmail.com>