mhi: core: use debugfs lookup method to find parent directory

Instead of storing debug_fs directory inode in a global data
structure, use a look up method to get the directory inode.

CRs-Fixed: 2258358
Change-Id: I2df480d7ea29b1cc90dff3a7fa19bb9c46132552
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>