mhi: core: initialize MHI lpm_mutex before use

Initialize the mutex to avoid null ptr access during
usage.

CRs-Fixed: 2432460
Change-Id: Iedc6152b9616b825e31efab61d699a613cc73ac9
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>