spi: bfin_spi: Use DIV_ROUND_UP instead of open-coded

Use DIV_ROUND_UP to simplify the code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>