3c59x: balance page maps and unmaps

debug kernel noticed a screw up in 3c59x.  skbs being mapped as page were being
unmapped as singles.  Easy fix.  Tested by myself

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: Steffen Klassert <klassert@mathematik.tu-chemnitz.de>
Signed-off-by: David S. Miller <davem@davemloft.net>