mtd: nand: fix memory leak in ONFI extended parameter page

This fixes a memory leak in the ONFI support code for detecting the
required ECC levels from this commit:

  commit 6dcbe0cd
  Author: Huang Shijie <b32955@freescale.com>
  Date:   Wed May 22 10:28:27 2013 +0800

      mtd: get the ECC info from the Extended Parameter Page

In the success case, we never freed the 'ep' buffer.

Also, this fixes an oversight in the same commit where we (harmlessly)
freed the NULL pointer.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>