drivers: sensor: adi: adxl372: Remove logically dead code

Remove logically dead if and else-if conditions as shown by the static
analysis, replacing with the else statement.

When data_opt is assigned to the MIN of fifo_wmark_cfg and fifo_full_cfg,
both those variables will be non-NULL as an earlier condition ensures the
function returns if either one of the variables is NULL before assigning
to data_opt.

Signed-off-by: Alexander Apostolu <apostolu240@gmail.com>