mtd: ubi: Add logging functions ubi_msg, ubi_warn and ubi_err

Using logging functions instead of macros can reduce overall object size.

$ size drivers/mtd/ubi/built-in.o*
   text	   data	    bss	    dec	    hex	filename
 271620	 163364	  73696	 508680	  7c308	drivers/mtd/ubi/built-in.o.allyesconfig.new
 287638	 165380	  73504	 526522	  808ba	drivers/mtd/ubi/built-in.o.allyesconfig.old
  87728	   3780	    504	  92012	  1676c	drivers/mtd/ubi/built-in.o.defconfig.new
  97084	   3780	    504	 101368	  18bf8	drivers/mtd/ubi/built-in.o.defconfig.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Richard Weinberger <richard@nod.at>