mtd: spi-nor: Fix integer overflow in stacked memories support

mtd: spi-nor: Fix integer overflow in stacked memories support

The 5d40b3d384dc ("mtd: spi-nor: Add parallel and stacked memories support")
adds new SPI bus flags, but also introduces a completely new set of
SPI bus flags in another location. The existing flags field is type
u8, while the new separate flags are BIT(8) and higher. Use of those
new flags triggers integer overflow.

Drop the newly introduced flags which were never used anywhere in the
code. Move the one remaining flag which was used in the correct place
and change it from BIT(8) to BIT(6) so it fits the u8 flags.

Fixes: 5d40b3d384dc ("mtd: spi-nor: Add parallel and stacked memories support")
Addresses-Coverity-ID: 510804 Extra high-order bits
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>