dmaengine: dmatest: Explicitly cast divisor to u32

dmaengine: dmatest: Explicitly cast divisor to u32

As the comment explains, the while loop ensures that runtime fits into
32 bits. Since do_div() casts the divisor to u32 anyway, explicitly cast
runtime to u32 to remove the following Coccinelle/coccicheck warning
reported by do_div.cocci:

  WARNING: do_div() does a 64-by-32 division, please consider using div64_s64 instead

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Link: https://lore.kernel.org/r/20240711132001.92157-1-thorsten.blum@toblux.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>