usb: phy: ab8500-usb: convert to devm_kzalloc

Convert local data allocation to devm_kzalloc and drop unnecessary fail
path code.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>