spi: fsl_qspi: Move AHB read buffer config after LUT

spi: fsl_qspi: Move AHB read buffer config after LUT

When using CONFIG_FSL_QSPI_AHB_FULL_MAP the fsl_qspi_default_setup() sets
the BFGENCR register to use the LUT(SEQID_LUT_AHB) before the Look Up Table
is populated.

This result in a situation that after 'sf probe' command any memory
read from qspi using AHB will result in undefined behaviour (hang) untill
first 'sf read' op is executed.

Move the BFGENCR write to fsl_qspi_prepare_lut() to ensure that the setup
is consistent. AHB reads will use the default LUT(index 0) setup by previous
boot stage untill the first read op.

Signed-off-by: Pawel Kochanowski <pkochanowski@sii.pl>