spi: pxa2xx: Setup maximum supported DMA transfer length

When the commit b6ced294

   ("spi: pxa2xx: Switch to SPI core DMA mapping functionality")

switches to SPI core provided DMA helpers, it missed to setup maximum
supported DMA transfer length for the controller and thus users
mistakenly try to send more data than supported with the following
warning:

  ili9341 spi-PRP0001:01: DMA disabled for transfer length 153600 greater than 65536

Setup maximum supported DMA transfer length in order to make users know
the limit.

Fixes: b6ced294 ("spi: pxa2xx: Switch to SPI core DMA mapping functionality")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org