spi: zynq-qspi: Fix bits_per_word handling

The Zynq QSPI driver supports only 8-bit transfers
(which is exactly what is required by flash drivers),
not 32-bit transfers.Note that bits_per_word does not refer to
the width of the TXD and RXD registers, but to the alignment
and byte order of tx_buf and rx_buf;see the comment for
'struct spi_transfer' in include/linux/spi/spi.h.
*   Check qspi->bits_per_word in xqspips_setup().
*   Check transfer->bits_per_word in xqspips_work_queue().
*   Remove the check in xqspips_transfer().

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Reviewed-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>