mtd: nand: create a helper verification function

... verification for 'nand_erase_nand'

These checks are expected to be used by 'nand_lock' and 'nand_unlock'
routines too. As all these three are block aligned operations.
So, creating a helper function for this makes sense.

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>