rmnet_shs: Add rmnet_mhi0 phy_dev support

Before rmnet_shs would not become active if non rmnet_ipa0
physical device was registered for rmnet_driver. This change
adds support for a rmnet_mhi physical device.

CRs-Fixed: 2438803
Change-Id: I8f5084e712d8e4456804add7777b4d9771498104
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>