um/time: Fixup namespace collision

The new timer_setup() function for struct timer_list collides with a
private um function. Rename it.

Fixes: 686fef92 ("timer: Prepare to change timer callback argument type")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Weinberger <richard@nod.at>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: user-mode-linux-devel@lists.sourceforge.net
Cc: Kees Cook  <keescook@chromium.org>