PCI/P2PDMA: Clean up documentation and kernel-doc

Fix typos, spellos, and grammar in p2pdma.rst and p2pdma.c.

Fix return value(s) in function pci_p2pmem_alloc_sgl().

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Jonathan Corbet <corbet@lwn.net>