xen: remove redundant initialization of variable ret

After commit 9f51c05d

 ("pvcalls-front: Avoid
get_free_pages(GFP_KERNEL) under spinlock"), the variable ret is being
initialized with '-ENOMEM' that is meaningless. So remove it.

Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Link: https://lore.kernel.org/r/20200919031702.32192-1-jingxiangfeng@huawei.com
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>