MFD: ti_am335x_tscadc: Pass correct error message

Pass on the correct error message from platform_get_irq()
instead of hard coding it to "EINVAL".

Also change label from "err" to "ret" for better
readability and update the same in error path.

Signed-off-by: Patil, Rachna <rachna@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>