xen-blkfront: Fix 'physical' typos

Fix misspelling of "physical".

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20210126205509.2917606-1-helgaas@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>