mhi: Use standard kernel return codes

Replace the MHI defined enum for return codes with
kernel standard return codes. This greatly improves readability
and parsing of errors by userspace clients.

Change-Id: Ica47f8c3a149f23243b5fb6a4aabcdc0a36b1535
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>
Signed-off-by: Tony Truong <truong@codeaurora.org>