Consistently use pdev as the variable of type struct pci_dev *.

Update DMA mapping documentation to use 'pdev' rather than 'dev' in
example code that calls routines expecting 'struct pci_device *', since 'dev'
might make readers think they're passing 'struct device *' parameters.
Bug 10397.

Signed-off-by: Matti Linnanvuori <mattilinnanvuori@yahoo.com>
Acked-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>