net: usb: rmnet_usb_data: Disable too much logspam

Many "rmnet_usb 1-1:1.6: [rmnet_usb0] error: rmnet_ioct
called for unsupported cmd" messages are output when doing
Embedded Rmnet communication. It is caused by receiving
unsupported commands in rmnet_ioctl function. This is not
an error condition, so downgrade to debug log level to quiet
messages appearing on console. Moreover, Change %d to %x
in this dev_dbg function call. %x will make it easier to
identify ioctl code since they are usually defined in hex
format in the header files.

Change-Id: I776e9ced0a8a042331d73ab0cfebacc33f61dfc6
CRs-fixed: 622875
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>