ARM: OMAP: fix typo "CONFIG_SMC91x_MODULE"

There's a (rather subtle) typo in "CONFIG_SMC91x_MODULE". Fix it once
and for all by using IS_ENABLED(), which is designed to avoid issues
like this.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tony Lindgren <tony@atomide.com>