pxa168fb: remove set but not used variables 'mi'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/video/fbdev/pxa168fb.c: In function 'pxa168fb_set_par':
drivers/video/fbdev/pxa168fb.c:408:29: warning:
 variable 'mi' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>