mhi: core: change link level lpm functions to return a code

When processing time critical events, it's important for core
driver to check for link level low power mode state before
continuing further. Change the lpm disable and enable functions
to return an error code when changing low power states.

CRs-Fixed: 2306423
Change-Id: Ida006ad56f9a549837debc973b8727f9cae8e84f
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>