drivers: spi: spi-bitbang: Fix CS glitch in transceive

spi_context_cs_configure_all() is currently called from
spi_bitbang_transceive(). This causes a glitch when combined with
SPI_HOLD_ON_CS is used.

Move the initialization to spi_bitbang_init which is what the other
SPI drivers seem to do.

Signed-off-by: Andreas Sandberg <andreas@sandberg.uk>