mfd: stmpe: Rid data size incompatibility warn when building for 64bit

Extinguishes:

../drivers/mfd/stmpe-i2c.c: In function ‘stmpe_i2c_probe’:
../drivers/mfd/stmpe-i2c.c:88:13:
	warning: cast from pointer to integer of different size
	partnum = (int)of_id->data;

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>