net: qualcomm: rmnet: Remove reference to extack

There is no existing definition of struct netlink_ext_ack for
ndo_add_slave, so remove this reference.

CRs-Fixed: 2156182
Change-Id: I42b86dc004b6b4aa0f4aed5a0ea18fe3afc82ac3
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>