drivers: char: mem: Simplify DEVKMEM configuration

Simplify the use of CONFIG_DEVKMEM by making the kmem_fops
so that it is __maybe_unused.

This enabled the multiple #ifdef's used for this structure
to be removed and brings it in line with the use of CONFIG_DEVMEM

This change should introduce no functional changes.

Signed-off-by: Rob Ward <robert.ward114@googlemail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>