nand/onenand: Fix missing argument checking for "markbad" command

The "nand markbad" and "onenand markbad" commands did not check if an
argument was passed; if this was forgotten, no error was raised but
block 0 was marked as bad.

While fixing this bug, clean up the code a bit and allow to pass more
than one block address, thus allowing to mark several blocks as bad
in a single command invocation.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>