EHF: Fix priority check

When deactivating, it's not an error if the priority being deactivating
is equal to the active priority. Fix this.

Change-Id: I66f0e9e775ac9aba8a7cc48cd3ecd3b358be63c0
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>