spi: zynq-qspi: Fix BAUD_RATE_DIV calculation

In xqspips_setup_transfer(), loop while baud_rate_val < 7, not < 8;
otherwise, we may end up with BAUD_RATE_DIV = 8, which is invalid.

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>