mtd: spi-nor: swp: Use a pointer for SR instead of a single byte

mtd: spi-nor: swp: Use a pointer for SR instead of a single byte

At this stage, the Status Register is most often seen as a single
byte. This is subject to change when we will need to read the CMP bit
which is located in the Control Register (kind of secondary status
register). Both will need to be carried.

Change a few prototypes to carry a u8 pointer. This way it also makes it
very clear where we access the first register, and where we will access
the second.

There is no functional change.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>