ravb: Use unsigned int for num_tx_desc variable in struct ravb_private

The number of TX descriptors per packet is an unsigned value and
the variable for holding this information should be unsigned.

This patch replaces the data type of num_tx_desc variable in struct
ravb_private from 'int' to 'unsigned int'.
This patch also updates the data type of local variables to unsigned int,
where the local variables are evaluated using num_tx_desc.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>