mhi: dev: netdev: Update logic for valid access of dentry variable

Variable "dentry" is valid only if "CONFIG_DEBUG_FS" is set.
Update logic such that "dentry" variable is accessed during
debugfs removal only in valid condition.

Change-Id: I2a1e3e7f312cf316c94e930e15e9528fbeedb9a1
Signed-off-by: Nitesh Gupta <nitegupt@codeaurora.org>