fpga: zynqpl: fix buffer alignment

Due to pointer arithmetic, "sizeof(u32) * ARCH_DMA_MINALIGN" is
subtracted. It seems that the original intention was to just subtract
ARCH_DMA_MINALIGN. Fix it.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>