mhi: core: Call vfree instead of kfree

mhi channel struct pointer is allocated using vzalloc. This pointer
is freed using kfree, which causes page fault. Fix this issue by
calling vfree.

Change-Id: Ibead85231c5cfd3f2e3f9776fdbbd4a9e4163311
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>