net: atlantic: Signedness bug in aq_vec_isr_legacy()

irqreturn_t type is an enum and in this context it's unsigned, so "err"
can't be irqreturn_t or it breaks the error handling.  In fact the "err"
variable is only used to store integers (never irqreturn_t) so it should
be declared as int.

I removed the initialization because it's not required.  Using a bogus
initializer turns off GCC's uninitialized variable warnings.  Secondly,
there is a GCC warning about unused assignments and we would like to
enable that feature eventually so we have been trying to remove these
unnecessary initializers.

Fixes: 7b0c342f ("net: atlantic: code style cleanup")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>