mtd: spi-nor: use is_power_of_2()

There is already a function to check if an integer is a power of 2. Use
it.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20210305234552.19204-1-michael@walle.cc