net: rtl8169: Fix DMA minimal aligned compile warning in RISC-V

For RISC-V architeture, hardware maintain the dcache coherency.
Software do not flush the cache. So even cache-line size larger
than descriptor size, driver can work.

Signed-off-by: Minda Chen <minda.chen@starfivetech.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>