staging: iio: ade7759: fix signed extension bug on shift of a u8

The current shift of st->rx[2] left shifts a u8 24 bits left,
promotes the integer to a an int and then to a unsigned u64. If
the top bit of st->rx[2] is set then we end up with all the upper
bits being set to 1. Fix this by casting st->rx[2] to a u64 before
the 24 bit left shift.

Detected by CoverityScan CID#144940 ("Unintended sign extension")

Fixes: 2919fa54 ("staging: iio: meter: new driver for ADE7759 devices")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/meter/ade7759.c

@@ -172,7 +172,7 @@ static int ade7759_spi_read_reg_40(struct device *dev,
 				reg_address);
 		goto error_ret;
 	}
-	*val = ((u64)st->rx[1] << 32) | (st->rx[2] << 24) |
+	*val = ((u64)st->rx[1] << 32) | ((u64)st->rx[2] << 24) |
 		(st->rx[3] << 16) | (st->rx[4] << 8) | st->rx[5];
 
 error_ret: