SMBIOS: Correct length calculation for empty string table

If all strings in SMBIOS table are empty, smbios_string_table_len
function should return 2, cause every table must end with "\0\0".

Also replace "eos" field type in smbios structures
from char to u8.

Change-Id: Ia3178b0030aa71e1ff11a3fd3d102942f0027eb1
Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
Reviewed-on: https://review.coreboot.org/20840
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/drivers/intel/wifi/wifi.c

@@ -40,7 +40,7 @@ static int smbios_write_wifi(struct device *dev, int *handle,
 		u8 length;
 		u16 handle;
 		u8 str;
-		char eos[2];
+		u8 eos[2];
 	} __packed;
 
 	struct smbios_type_intel_wifi *t =