SPI: zynq: Corrected upper limit for baud_rate_val in loop

In the loop where baud rate divisor value to be written to the
config register is calculated, the check was "baud_rate_val < 8"
which resulted in baud rate divisor field being set to '000'
which is invalid. Changed check to "baud_rate_val < 7".

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