serial: omap: delete .set_wake callback

This callback is unused by the serial core since pre-git days
and is not coming back. Delete it. Enabling wakeup on the
OMAP serial driver is done through other runpaths these days.

Cc: Tony Lindgren <tony@atomide.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>