arm: omap2plus: fix ads7846 pendown gpio request

introduced by: 96974a24
(omap: consolidate touch screen initialization among different boards)

ads7846 driver can use either gpio_pendown or get_pendown_state()
callback. In case of gpio_pendown, it requests the provided gpio_pendown
thus resulting in double requesting that gpio:

ads7846 spi1.0: failed to request pendown GPIO57
ads7846: probe of spi1.0 failed with error -16

Fix this by restricting the gpio request to the case of
get_pendown_state() callback is used.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Tested-by: Thomas Weber <weber@corscience.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>