spi: pxa2xx: Get rid of unused enable_loopback member

There is no user of the enable_loopback member in the struct pxa2xx_spi_chip.
Remote this legacy member completely.

The mentioned in the documentation the testing phase can be performed with
spidev_test tool.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211123192723.44537-3-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>