Use DIV_ROUND_UP() in cfi_udelay() instead of open-coding it...

Doing this also helpfully gets rid of two complaints from
'scripts/checkpatch.pl --strict':

CHECK: spaces preferred around that '+' (ctx:VxV)
#29: FILE: drivers/mtd/chips/cfi_util.c:29:
+		msleep((us+999)/1000);
 		          ^

CHECK: spaces preferred around that '/' (ctx:VxV)
#29: FILE: drivers/mtd/chips/cfi_util.c:29:
+		msleep((us+999)/1000);
			       ^
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>