mtd: spi-nor: micron-st: Skip FSR reading if SPI controller does not support it

The Intel SPI controller does not support low level operations, like
reading the flag status register (FSR). It only exposes a set of high
level operations for software to use. For this reason check the return
value of micron_st_nor_read_fsr() and if the operation was not
supported, use the status register value only. This allows the chip to
work even when attached to Intel SPI controller (there are such systems
out there).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220506105158.43613-1-mika.westerberg@linux.intel.com