msm: mhi_dev: Fail start command if fails to allocate reqs

Fail the start command if we fails to allocate buffer for event
requests.

Change-Id: I05e9acb093f53f1675a4f711182730ab420e0e5a
Signed-off-by: Veerabhadrarao Badiganti <quic_vbadigan@quicinc.com>