net: smc911x: avoid unused variable warnings

The change to use the generic DMA engine API in the smc911x
driver has led to a harmless warning about unused local variables:

smsc/smc911x.c: In function 'smc911x_probe':
smsc/smc911x.c:1796:20: error: unused variable 'param'
smsc/smc911x.c:1795:17: error: unused variable 'mask'
smsc/smc911x.c:1794:26: error: unused variable 'config'

This puts the variable declarations inside of the same #ifdef
that protects their use.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 79d3b59a ("net: smc911x: convert pxa dma to dmaengine")
Signed-off-by: David S. Miller <davem@davemloft.net>