During a transfer the driver filled the fifo with 4bytes,
even if the data that needs to be transfer is less that 4bytes.
This resulted in slab-out-of-bounds bug in KernelAddressSanitizer.

This patch resolves slab-out-of-bounds bug by filling the fifo
with the number of bytes that needs to transferred.
Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Link: https://lore.kernel.org/r/20210416004652.2975446-4-quanyang.wang@windriver.comSigned-off-by: Mark Brown <broonie@kernel.org>