atl1[ce]: fix sparse warnings

The dmaw_block is an enum and max_pay_load is u32. Therefore
sparse gives warning about comparison of unsigned and signed value.
Resolve by using min_t to force cast.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>