mhi: dev: netdev: make chaining debugfs function static

Add missing static keyword to debugfs function for disabling
chaining of SKBs.

CRs-Fixed: 2482422
Change-Id: I0bc9fb8adcec2b60338d4d0f59caa4df498f0b43
Signed-off-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>