usb: gadget: u_rmnet_ctrl_qti: Disable too much logspam

Many "rmnet file handler ef69a580(index=0) is not open"
messages are printed during normal operation. It is caused
by sending packet to modem when the rmnet device control
port is not open. This is not an error condition, so downgrade
to debug log level to quiet messages appearing on console.

Change-Id: Iffb0925436f394df497b176684387130d60574ed
CRs-fixed: 610106
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>