platform/x86: hp-wmi: Use DEVICE_ATTR_(RO|RW) helper macros

Use the DEVICE_ATTR_(RO|RW) macros, ranaming the show and store
functions accordingly.

Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>
Tested-by: Carlo Caione <carlo@endlessm.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>