ARM: imx: leave irq_base of wm8350_platform_data uninitialized

With commit d1738aef (mfd: Allocate wm835x irq descs dynamically) being
in the tree, there is no need to initialize irq_base field of struct
wm8350_platform_data.  Remove it to save one reference to macro
MXC_BOARD_IRQ_START.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>