mfd: bcm590xx: Fix type argument for module device table

This fixes below build error.

FATAL: drivers/mfd/bcm590xx: sizeof(struct i2c_device_id)=24 is not a modulo of the size of section __mod_i2c_device_table=392.
Fix definition of struct i2c_device_id in mod_devicetable.h
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>