video: fbdev: omapfb: lcd_ams_delta: fix unused variable warning

A recent cleanup patch removed the only reference to a local variable
in some configurations.

Move the variable into the one block it is still used in, inside
of an #ifdef, to avoid this warning.

Fixes: 9d773f10 ("video: fbdev: omapfb: lcd_ams_delta: Make use of the helper function dev_err_probe()")
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>