msm: rmnet: do not close the SMD port during close IOCTL

When the close IOCTL is called, the driver currently closes the
underlying SMD port. This results in sending DTR drop to the modem thus
closing all QMI clients open on that port.

Change the close behavior to be localized within RMNET driver.

CRs-Fixed: 393562
Change-Id: Ib7f25b35acfb4058018a82337c482dcdd333bf41
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>