usb: gadget: f_qc_rndis: Fix double-free in qcrndis_free_inst

qcrndis_free_inst function can double free f_qc_rndis pointer.
Hence fix this memory bug in qcrndis_free_inst function.

Same bool RNDIS flag is used for USB_CONFIGFS_RNDIS and
USB_CONFIGFS_QCRNDIS. Add bool Change in Kconfig to differentiate
these two different configs.

Change-Id: I8e7c4be090107618cd6cbac394a57f109f8a1ced
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>