vme: vme_get_size potentially returning incorrect value on failure

The function vme_get_size returns the size of the window to the caller,
however it doesn't check the return value of the call to vme_master_get.

Return 0 on failure rather than anything else.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>