swiotlb: fix implicit debugfs declarations

Factor out the debugfs bits from rmem_swiotlb_device_init() into a separate
rmem_swiotlb_debugfs_init() to fix the implicit debugfs declarations.

Fixes: 461021875c50 ("swiotlb: Add restricted DMA pool initialization")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Claire Chang <tientzu@chromium.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>