xen/xenbus: Convert to use ERR_CAST()

Use ERR_CAST() as it is designed for casting an error pointer to
another type.

This macro utilizes the __force and __must_check modifiers, which instruct
the compiler to verify for errors at the locations where it is employed.

Signed-off-by: Shen Lichuan <shenlichuan@vivo.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20240829084710.30312-1-shenlichuan@vivo.com>
Signed-off-by: Juergen Gross <jgross@suse.com>