Ensure values are initialized in capabilities_get

To ensure no uninitialized variables are used, the hw and id structs are
now zero initialized and the result of the conversion to a direct drive
device is checked.

Change-Id: Ic1f0e4685bcd56d82d4eb8b0d2b0d0d7a18fc543
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>