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 <christophe.jaillet@wanadoo.fr>
Acked-by: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/aa641732ba707ce3690217825c3ca7373ffde4f9.1748191985.git.christophe.jaillet@wanadoo.fr