spi: cf_qspi: Use DIV_ROUND_UP at appropriate place

This change slightly improves readability.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Richard Retanubun <richardretanubun@ruggedcom.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>