Do not initialize the flags field in struct gpiobus_pin from the flags in
struct gpio_pin. It turns out these two sets of flags are completely
unrelated to each other.

Also, update the comment for GPIO_ACTIVE_LOW to reflect the fact that it
does get set, somewhat unobviously, by code that parses FDT data. The bits
from the FDT cell containing flags are just copied to gpiobus_pin.flags, so
there's never any obvious reference to the symbol GPIO_ACTIVE_LOW being
stored into the flags field.