mtd: spi-nor: Constify struct spi_nor_fixups

'struct spi_nor_fixups' are not modified in this driver.

Constifying these structures moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig, as an example:
Before:
======
text data bss dec hex filename
23304 13168 0 36472 8e78 drivers/mtd/spi-nor/micron-st.o

After:
=====
text data bss dec hex filename
23560 12912 0 36472 8e78 drivers/mtd/spi-nor/micron-st.o

Signed-off-by: Christophe JAILLET <[e-mail]>
Acked-by: Pratyush Yadav <[e-mail]>
Signed-off-by: Pratyush Yadav <[e-mail]>
Link: https://lore.kernel.org/r/aa641732ba[e-mail]

drivers/mtd/spi-nor/micron-st.c

···
 	return 0;
 }
 
-static struct spi_nor_fixups mt25qu512a_fixups = {
+static const struct spi_nor_fixups mt25qu512a_fixups = {
 	.post_bfpt = mt25qu512a_post_bfpt_fixup,
 };
 
···
 	return spi_nor_set_4byte_addr_mode(nor, true);
 }
 
-static struct spi_nor_fixups n25q00_fixups = {
+static const struct spi_nor_fixups n25q00_fixups = {
 	.late_init = st_nor_four_die_late_init,
 };
 
-static struct spi_nor_fixups mt25q01_fixups = {
+static const struct spi_nor_fixups mt25q01_fixups = {
 	.late_init = st_nor_two_die_late_init,
 };
 
-static struct spi_nor_fixups mt25q02_fixups = {
+static const struct spi_nor_fixups mt25q02_fixups = {
 	.late_init = st_nor_four_die_late_init,
 };
 

drivers/mtd/spi-nor/spansion.c

···
 	return 0;
 }
 
-static struct spi_nor_fixups s25fs256t_fixups = {
+static const struct spi_nor_fixups s25fs256t_fixups = {
 	.post_bfpt = s25fs256t_post_bfpt_fixup,
 	.post_sfdp = s25fs256t_post_sfdp_fixup,
 	.late_init = s25fs256t_late_init,
···
 	return 0;
 }
 
-static struct spi_nor_fixups s25hx_t_fixups = {
+static const struct spi_nor_fixups s25hx_t_fixups = {
 	.post_bfpt = s25hx_t_post_bfpt_fixup,
 	.post_sfdp = s25hx_t_post_sfdp_fixup,
 	.late_init = s25hx_t_late_init,