mtd: rawnand: fsmc: Change to non-atomic bit operations

No need to use expensive atomic change_bit() on dat[] and err_idx[]:
1. fsmc_bch8_correct_data() is called while mutex chip->lock is held
2. err_idx[] is a local variable.

To avoid big endian concern due to type cast to unsigned long, directly
change the bit in the specified byte instead of using non-atomic
__change_bit().

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1576886755-9788-1-git-send-email-fenghua.yu@intel.com