sensors: lsm6dsv16x: fix get_frame_count API

The get_frame_count() API must return the number of frames in FIFO
for a given channel type, and not the total number of generic FIFO
frames. This API may be also re-used in the decoder routine when
the timestamp baseline is adjusted.

Signed-off-by: Armando Visconti <armando.visconti@st.com>