usb: rndis: Rename CONFIG_RNDIS_TX_BUF_* and CONFIG_RNDIS_CMD_BUF_*

Rename CONFIG_RNDIS_TX_BUF_* to CFG_RNDIS_TX_BUF_* and rename
CONFIG_RNDIS_CMD_BUF_* to CFG_RNDIS_CMD_BUF_*.  The CONFIG options where
not exposed in Kconfig so limit use of CONFIG_ to Kconfig only symbols.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>