pinctrl: mcp23s08: Read spi-present-mask as u8 not u32

pinctrl: mcp23s08: Read spi-present-mask as u8 not u32

The binding (microchip,mcp23s08) specifies microchip,spi-present-mask
as uint8, but driver would read u32, causing type mismatch. Use
device_property_read_u8 to match binding spec, hardware (8 chips max),
& prevent probe failure.

Cc: stable@vger.kernel.org
Fixes: 3ad8d3ec6d87 ("dt-bindings: pinctrl: convert pinctrl-mcp23s08.txt to yaml format")
Signed-off-by: Judith Mendez <jm@ti.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>