ipc: Handle error conditions in ipc command

Handle error conditions in intel_scu_ipc_command() and
pwr_reg_rdwr().

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>