uml: kill a kconfig warning

Got this warning from Kconfig:

   boolean symbol INPUT tested for 'm'? test forced to 'n'

because INPUT is tristate, not bool.

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>