net: flexcan: convert to clk_prepare/clk_unprepare

The patch converts flexcan driver to clk_prepare/clk_unprepare by
using helper functions clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: David S. Miller <davem@davemloft.net>