mfd: tps65912: Fix variable name for SPI remove

The SPI interface is mostly a copy/paste from the I2C interface with some
minor renaming for consistency. "Client" is an I2C specific term that
was left in the SPI remove path, rename this here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>