xen-netback: fix error handling output

xen-netback: fix error handling output

The connect function prints an unintialized error code after an
earlier initialization was removed:

drivers/net/xen-netback/xenbus.c: In function 'connect':
drivers/net/xen-netback/xenbus.c:938:3: error: 'err' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This prints it as -EINVAL instead, which seems to be the most
appropriate error code. Before the patch that caused the warning,
this would print a positive number returned by vsscanf() instead,
which is also wrong. We probably don't need a backport though,
as fixing the warning here should be sufficient.

Fixes: f95842e7a9f2 ("xen: make use of xenbus_read_unsigned() in xen-netback")
Fixes: 8d3d53b3e433 ("xen-netback: Add support for multiple queues")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>