mhi: core: fix returning incorrect code when entering rddm mode

Exit from mhi_force_rddm_mode returns failure in success case, fix
return code.

CRs-Fixed: 2336233
Change-Id: I6b5d7134e6c32b788dfb2f2ffb329079945bb5d1
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>