mtd: aspeed: fix compile warning in aspeed_smc_read_from_ahb()

The first argument of ioread32_rep() and ioread8_rep is not
const. Change aspeed_smc_read_from_ahb() prototype to fix compile
warning :

   drivers/mtd/spi-nor/aspeed-smc.c: In function 'aspeed_smc_read_from_ahb':
   drivers/mtd/spi-nor/aspeed-smc.c:212:16: warning: passing argument 1 of 'ioread32_rep' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      ioread32_rep(src, buf, len >> 2);

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>