staging: r8188eu: use sizeof instead of hardcoded firmware header size

Use sizeof() instead of hardcoding the firmware header size and add
a compile time check to ensure struct rt_firmware_hdr has the correct
size.
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20220417175441.13830-6-straube.linux@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/core/rtw_fw.c

@@ -39,6 +39,8 @@ struct rt_firmware_hdr {
 	__le32	rsvd5;
 };
 
+static_assert(sizeof(struct rt_firmware_hdr) == 32);
+
 static void fw_download_enable(struct adapter *padapter, bool enable)
 {
 	u8 tmp;
@@ -268,9 +270,8 @@ int rtl8188e_firmware_download(struct adapter *padapter)
 			DRIVER_PREFIX, fw_version, fw_subversion, fw_signature);
 
 	if (IS_FW_HEADER_EXIST(fwhdr)) {
-		/*  Shift 32 bytes for FW header */
-		fw_data = fw_data + 32;
-		fw_size = fw_size - 32;
+		fw_data = fw_data + sizeof(struct rt_firmware_hdr);
+		fw_size = fw_size - sizeof(struct rt_firmware_hdr);
 	}
 
 	/*  Suggested by Filen. If 8051 is running in RAM code, driver should inform Fw to reset by itself, */