spi: spi-fsl-dspi: Fix copy-paste error in dspi_probe

It seems that the proper structure field to use in this particular
case is *regmap_pushr* instead of regmap.

Addresses-Coverity-ID: 1470126 ("Copy-paste error")
Fixes: 58ba07ec ("spi: spi-fsl-dspi: Add support for XSPI mode registers")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Esben Haabendal <eha@deif.com>
Signed-off-by: Mark Brown <broonie@kernel.org>