swiotlb: Group identical cleanup in swiotlb_cleanup()

Avoid repeating the zeroing of global swiotlb variables in two locations
and introduce swiotlb_cleanup() to do that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>