adaasync(): Set ADA_STATE_WCACHE based on ADA_FLAG_CAN_WCACHE

The attached patch lets adaasync() set ADA_STATE_WCACHE based on
ADA_FLAG_CAN_WCACHE instead of ADA_FLAG_CAN_RAHEAD.

This fixes a regression introduced in r300207 which changed
the flag names.

PR: 220948
Submitted by: Fabian Keil <fk@fabiankeil.de>
Obtained from: ElectroBSD
MFC after: 1 week