swiotlb: drop pointless static qualifier in swiotlb_create_debugfs()

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

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>