mhi: core: confirm MHI offset is valid before using it

PCIe link can go down anytime asynchronously without host
knowing. In event of a link-down, MMIO reads will always
return 0xffffffff.  Confirm the read value is valid prior
to executing further.

CRs-Fixed: 1030692
Change-Id: If052a1a31c62a81f380cec3f523b605b38a33847
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>