mfd: htc-i2cpld: Rectify pointer offset error

Checking the result of container_of() against NULL will always result to
false.  Using the offset of member 'chip_out' to find the start of 'struct
htcpld_chip' will result in an offset error when .get_chip() is attempting
to obtain 'htcpld-in'.  Instead, we'll use the correct member based on a
previously the set chip label.

Signed-off-by: Lee Jones <lee.jones@linaro.org>