platform: make better use of to_platform_{device,driver}() macros

This helps the code look more consistent and cleaner.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>