[MMC] Improve error handling of Samsung e-MMC wear leveling quirk

Prior to this change, errors when reading from the e-MMC during
the handling of the quirk would not be detected.

In addition, if an error did occur, the actual error code would
be overwritten by the code that exits the special OEM mode.

Change-Id: Ic470cf8ef4edb27119f0fdf1fa1b0cf44a2bf31e
Signed-off-by: Scott Anderson <saa@android.com>