mtd: rawnand: Do not check for bad block if bbt is unavailable

The bbt pointer will be unavailable when NAND_SKIP_BBTSCAN option is
set for a NAND chip. The intention is to skip scanning for the bad
blocks during boot time. However, the MTD core will call
_block_isreserved() and _block_isbad() callbacks unconditionally for
the rawnand devices due to the callbacks always present while collecting
the ecc stats.

The _block_isreserved() callback for rawnand will bail out if bbt
pointer is not available. But _block_isbad() will continue without
checking for it. So this contradicts with the NAND_SKIP_BBTSCAN option
since the bad block check will happen anyways (ie., not much difference
between scanning for bad blocks and checking each block for bad ones).

Hence, do not check for the bad block if bbt pointer is unavailable.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>