Fix computation in nand_util.c:get_len_incl_bad

Depending on offset, flash size and the number of bad blocks,
get_len_incl_bad may return a too small value which may lead to:

1) If there are no bad blocks, nand_{read,write}_skip_bad chooses the
bad block aware read/write code. This may hurt performance, but does
not have any adverse effects.

2) If there are bad blocks, the nand_{read,write}_skip_bad may choose
the bad block unaware read/write code (if len_incl_bad == *length)
which leads to corrupted data.

Signed-off-by: Daniel Hobi <daniel.hobi@schmid-telecom.ch>