mfd: stmpe: Get rid of irq_invert_polarity

Since the very first patch, stmpe core driver is using irq_invert_polarity as
part of platform data. But, nobody is actually using it in kernel till now.

Also, this is not something part of hardware specs, but is included to cater
some board mistakes or quirks.

So, better get rid of it. This is earlier discussed here:

https://lkml.org/lkml/2012/11/27/636



Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>