video: fbdev: sm712fb.c: fixed prefer unsigned int warning

Fixed several prefer 'unsigned int' over 'unsigned' warnings checked by
scripts/checkpatch.pl:
  Prefer 'unsigned int' to bare use of 'unsigned'

Signed-off-by: Lynn Lei <lynnl.wit@gmail.com>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>