atm: zatm: fix memcmp casting

BugLink: https://bugs.launchpad.net/bugs/1784382

[ Upstream commit f9c6442a ]

memcmp() returns int, but eprom_try_esi() cast it to unsigned char. One
can lose significant bits and get 0 from non-0 value returned by the
memcmp().
Signed-off-by: Ivan Bornyakov <brnkv.i1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/atm/zatm.c

@@ -1149,8 +1149,8 @@ static void eprom_get_byte(struct zatm_dev *zatm_dev, unsigned char *byte,
 }
 
 
-static unsigned char eprom_try_esi(struct atm_dev *dev, unsigned short cmd,
-				   int offset, int swap)
+static int eprom_try_esi(struct atm_dev *dev, unsigned short cmd, int offset,
+			 int swap)
 {
 	unsigned char buf[ZEPROM_SIZE];
 	struct zatm_dev *zatm_dev;