MIPS: Alchemy: fix xxs1500 build error

This fixes:
alchemy/xxs1500/init.c: In function 'prom_init':
alchemy/xxs1500/init.c:57:17: error: ignoring return value of 'kstrtoul', declared with attribute warn_unused_result
Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/2340/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/alchemy/xxs1500/init.c

@@ -51,10 +51,9 @@ void __init prom_init(void)
 	prom_init_cmdline();
 
 	memsize_str = prom_getenv("memsize");
-	if (!memsize_str)
+	if (!memsize_str || strict_strtoul(memsize_str, 0, &memsize))
 		memsize = 0x04000000;
-	else
-		strict_strtoul(memsize_str, 0, &memsize);
+
 	add_memory_region(0, memsize, BOOT_MEM_RAM);
 }