fpga manager: xilinx-spi: remove unneeded, mistyped variables

Using variables does not add readability here: parameters passed
to udelay*() are obviously in microseconds and their meaning is clear
from the context.

The type is also wrong, udelay expects an unsigned long.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Moritz Fischer <mdf@kernel.org>