net: ethernet: sun: Remove redundant code

Since the starting value in the for loop is greater than or equal to 1,
the restriction is CAS_FLAG_REG_PLUS is in the file cassini.h is
defined as 0x1 by macro, and the for loop and if condition is not
satisfied, so the code here is redundant.

Clean up the following smatch warning:

drivers/net/ethernet/sun/cassini.c:3513 cas_start_dma() warn: we never
enter this loop.

drivers/net/ethernet/sun/cassini.c:1239 cas_init_rx_dma() warn: we never
enter this loop.

drivers/net/ethernet/sun/cassini.c:1247 cas_init_rx_dma() warn: we never
enter this loop.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>