mtd: spi-nor: check FSR error bits for Micron memories

For Micron spi nor device, when erase/program operation
fails, especially the failure results from intending to
modify protected space, spi-nor upper layers still get
the return which shows the operation succeeds. This is
because current spi_nor_fsr_ready() only uses FSR bit.7
(flag status register) to check device whether ready.
This patch fixes this issue by checking relevant error
bits in FSR.
The FSR is a powerful tool to investigate the status of
device, checking information regarding what the memory is
actually doing and detecting possible error conditions.

Signed-off-by: beanhuo <beanhuo@micron.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>