xen/pcifront: Removed unnecessary __ref annotation

xen/pcifront: Removed unnecessary __ref annotation

An unnecessary "__ref" annotation was removed from the
"drivers/pci/xen_pcifront.c" file. The function where the annotation
was used was "pcifront_backend_changed()", which does not call any
functions annotated as "__*init" nor "__*exit". This makes "__ref"
unnecessary since this annotation is used to make the compiler ignore
section miss-matches when they are not happening here in the first
place.

In addition to the aforementioned change, some code style issues were
fixed in the same file.

Signed-off-by: Sergio Miguéns Iglesias <sergio@lony.xyz>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210830175305.13370-1-sergio@lony.xyz
Signed-off-by: Juergen Gross <jgross@suse.com>