iio: adc: at91_adc: Repair broken platform_data support

iio: adc: at91_adc: Repair broken platform_data support

Trying to use the at91_adc driver while not using device tree is ending up in a
kernel crash:

Unable to handle kernel NULL pointer dereference at virtual address 00000004
[...]
[<c01f3510>] (at91_adc_probe) from [<c0183828>] (platform_drv_probe+0x18/0x48)
[<c0183828>] (platform_drv_probe) from [<c01824a4>] (driver_probe_device+0x100/0x218)
[<c01824a4>] (driver_probe_device) from [<c0182648>] (__driver_attach+0x8c/0x90)
[<c0182648>] (__driver_attach) from [<c0180de4>] (bus_for_each_dev+0x58/0x88)
[<c0180de4>] (bus_for_each_dev) from [<c0181c7c>] (bus_add_driver+0xd4/0x1d4)
[<c0181c7c>] (bus_add_driver) from [<c0182c40>] (driver_register+0x78/0xf4)
[<c0182c40>] (driver_register) from [<c0008998>] (do_one_initcall+0xe8/0x14c)
[<c0008998>] (do_one_initcall) from [<c02f0b50>] (kernel_init_freeable+0xec/0x1b4)
[<c02f0b50>] (kernel_init_freeable) from [<c022acdc>] (kernel_init+0x8/0xe4)
[<c022acdc>] (kernel_init) from [<c0009670>] (ret_from_fork+0x14/0x24)

This is because the at91_adc_caps structure is mandatory but is not filled when
using platform_data. Correct that by using an id_table. It ensues that the
driver will not match "at91_adc" anymore but it was crashing anyway.

Fixes: c46016665fff (iio: at91: ADC start-up time calculation changed since at91sam9x5)
Cc: stable@vger.kernel.org # v3.13+
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>