platform/x86: fujitsu-laptop: remove pf_device field from struct fujitsu_bl

Both struct fujitsu_bl and struct fujitsu_laptop have a pf_device field.
Up until now the latter has been redundant, which is logically incorrect
because the primary function of the platform device created by
fujitsu-laptop is to provide laptop-related (not brightness-related)
attributes to userspace.

Remove the pf_device field from struct fujitsu_bl and make all module
code use the pf_device field of struct fujitsu_laptop instead.

Suggested-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Tested-by: Jonathan Woithe <jwoithe@just42.net>
Reviewed-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>