imx: only select HAVE_CAN_FLEXCAN if CAN is enabled

This fixes a warning

	warning: (IMX_HAVE_PLATFORM_FLEXCAN && ARCH_MXC) selects HAVE_CAN_FLEXCAN which has unmet direct dependencies (NET && CAN)

(The warning in general exists since

	246cf9c (kbuild: Warn on selecting symbols with unmet direct dependencies)

which was reverted between 71ebc01 and b1f7d6e.)

Reported-and-introduced-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>