Use DEVICE_ATTR_{RO,WO,RW} macros to simplify sysfs attributes
declaration.

To declare a "foo" attribute, DEVICE_ATTR_RW() requires foo_show() and
foo_store(), so rename a few functions to satisfy this requirement.

Also put the macro below each related show/store functions for clarity.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>