platform/x86: Simplify intel_mid_powerbtn

This patch:
1. Removes unnecessay #defines
2. Removes 'mfld_pb_priv' data structure which results in simpler error
   handling and less memory allocations.

Signed-off-by: Ameya Palande <2ameya@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>