bcm63xx: redboot: fix warning

drivers/mtd/parsers/redboot.c: In function 'parse_redboot_partitions':
drivers/mtd/parsers/redboot.c:194:59: warning: suggest parentheses around '-' in operand of '&' [-Wparentheses]
     fis_origin = (buf[i].flash_base & (master->size << 1) - 1);
                                       ~~~~~~~~~~~~~~~~~~~~^~~

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
(cherry picked from commit f314cbe54be4d90e748e18ae5e9fb00d5fd31c91)

target/linux/brcm63xx/patches-4.14/400-bcm963xx_flashmap.patch

@@ -40,7 +40,7 @@ Signed-off-by: Axel Gembe <ago@bastart.eu.org>
 +	} else {
 +		for (i = 0; i < numslots; i++) {
 +			if (!strncmp(buf[i].name, "RedBoot", 8)) {
-+				fis_origin = (buf[i].flash_base & (master->size << 1) - 1);
++				fis_origin = (buf[i].flash_base & ((master->size << 1) - 1));
 +			}
 +		}
 +	}