drivers/net: fix build warning in ethernet/sfc/tx.c

drivers/net: fix build warning in ethernet/sfc/tx.c

Commit ee45fd92c739db5b7950163d91dfe5f016af6d24 ("sfc: Use TX PIO
for sufficiently small packets") introduced the following warning:

drivers/net/ethernet/sfc/tx.c: In function 'efx_enqueue_skb':
drivers/net/ethernet/sfc/tx.c:432:1: warning: label 'finish_packet' defined but not used

Stick the label inside the same #ifdef that the code which calls
it uses.  Note that this is only seen for arch that do not set
ARCH_HAS_IOREMAP_WC, such as arm, mips, sparc, ..., as the others
enable the write combining code and hence use the label.

Cc: Jon Cooper <[email protected]>
Cc: Ben Hutchings <[email protected]>
Signed-off-by: Paul Gortmaker <[email protected]>
Acked-by: Shradha Shah <[email protected]>
Signed-off-by: David S. Miller <[email protected]>