[MTD] [NAND] Tidy up handling of page number in nand_block_bad()

Further to the previous patch fixing the calculation of page number,
both branches are using the same result. Clean up the function 
accordingly, calculating it (and also masking with pagemask) only in one 
place.

Signed-off-by: Thomas Knobloch <knobloch@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>