net: rmnet_data: Change the log level for unknown IOCTL's

Change the log level from high to medium when userspace calls an
unknown IOCTL on an rmnet data device. This log message is not
very useful and often causes log spam in the serial device
output which may occasionally lead to watchdog timeouts.

CRs-fixed: 1000453
Change-Id: I50e038d838eded30ee8304fefb2c13328eaf9683
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>