mtd: spi-nor: micron-st: convert USE_FSR to a manufacturer flag

Now that all functions using that flag are local to the micron module,
we can convert the flag to a manufacturer one.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Tested-by: Pratyush Yadav <p.yadav@ti.com> # on mt35xu512aba, s28hs512t
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-26-michael@walle.cc