mtd: spi-nor: swp: check SR_TB flag when getting tb_mask

mtd: spi-nor: swp: check SR_TB flag when getting tb_mask

When the chip does not support top/bottom block protect, the tb_mask
must be set to 0, otherwise SR1 bit5 will be unexpectedly modified.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Fixes: 3dd8012a8eeb ("mtd: spi-nor: add TB (Top/Bottom) protect support")
Reviewed-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>