powerpc/pseries: Drop pointless static qualifier in vpa_debugfs_init()

There is no need to have the 'struct dentry *vpa_dir' variable static
since new value always be assigned before use it.

Fixes: c6c26fb5 ("powerpc/pseries: Export raw per-CPU VPA data via debugfs")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190218125644.87448-1-yuehaibing@huawei.com