ath79: fix block protection clearing

ath79: fix block protection clearing

The block protection bits of macronix do not match the implementation.
The chip has 3 BP bits. Bit 5 is actually the third BP but here the
5th bit is SR_TB. Therefore the patch adds SR_TB to the mask. In the
4.19er kernel the whole register was simply set to 0.

The wrong implementation did not remove the block protection. This led
to jffs2 errors in the form of:
"jffs2: Newly-erased block contained word 0x19852003 at offset 0x..."
This caused inconsistent memory and other errors.

Suggested-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Nick Hainke <vincent@systemli.org>