msm: mhi_rmnet: remove headroom reservation

MHI_RMNET layer not required to reserve for IP headroom
data since rmnet_data layer will create a new skb if
headroom reservation is needed.

CRs-Fixed: 1085031
Change-Id: Ic04e73510140e6c00ee18107a8edc9ead0b53863
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>