usb: Fix compiler warnings

Fix two printk warnings in f_rndis and an incorrect call to
scnprintf() in u_ether_configfs.h.

Fixes: b42170b2 ("usb: Add support for rndis uplink aggregation")
Change-Id: Ic0dedbad1a970ddb76c3139fcf17efd2c86e6bd1
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>