SMC91x: Fix compilation on some platforms.

This reverts 51ac3bef ('SMC91x: delete
unused local variable "lp"') and adds __maybe_unused markers to these
(potentially) unused variables.

The issue is that in some configurations SMC_IO_SHIFT evaluates
to '(lp->io_shift)', but in some others it's plain '0'.

Based upon a build failure report from Manuel Lauss.

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