gpio: lp3943: Drop pin_used and lp3943_gpio_request/lp3943_gpio_free

The implementation of lp3943_gpio_request/lp3943_gpio_free test pin_used
for tracing the pin usage. However, gpiolib already checks FLAG_REQUESTED
flag for the same purpose. So remove the redundant implementation.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>