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

Commit ee45fd92 ("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 <jcooper@solarflare.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>