lpc24xx/lpc17xx: lpc24xx_pin_set_function() keep LPC4088 W type pin in digital mode for non-analog function.

The problem wit incorrect switching of pins into analog mode manifestes
on LPC4088 based board.

LPC4088 implements pin P1.17 (ENET_MDIO) as new W type (digital pin
with analog option). The pin was listed as D category on LPC1788
which does not have analog mode control bit. If analog option is
not explicitly switched off on LPC4088 then the pin does not work
as digital pin.

Code tested on LPC1788 and no problems has been observed even that
manual specifies the IOCON_ADMODE field as reserved and should
be written as zero. But even RTEMS lpc24xx_gpio_config sets this
bit unconditionally.

Signed-off-by: Pavel Pisa <ppisa@pikron.com>