ASoC: wm8940: fix a memory leak if wm8940_register return error

This patch adds checking for wm8940_register return value,
and does kfree(wm8940) if wm8940_register() fail.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>