igb: Fix warnings in igb_set_ringparam()

drivers/net/igb/igb_ethtool.c: In function ‘igb_set_ringparam’:
drivers/net/igb/igb_ethtool.c:744: warning: comparison of distinct pointer types lacks a cast
drivers/net/igb/igb_ethtool.c:748: warning: comparison of distinct pointer types lacks a cast

Casts were to u16 on the constant, but the type of new_{r,t}x_count is
u32.  Cast to u32 instead.

Signed-off-by: David S. Miller <davem@davemloft.net>