mtd: spi-nor: replace unnecessary div64_u64() with div_u64()

Both occurrences of div64_u64() just have a u8 or u32 divisor. Use
div_u64() instead. Many 32 bit architectures can optimize this variant
better than a full 64 bit divide.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/9ba7f4e6-2b8b-44a3-9cac-9ed6e50f1700@moroto.mountain/
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
[pratyush@kernel.org: touched up commit message]
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20240429121113.803703-1-mwalle@kernel.org