UBI: always start the background thread

This fix only affects UBI debugging.

If the the background thread is disabled for debugging purposes,
start it anyway, because otherwise we see tonns of kernel debugging
complaints like this:

INFO: task ubi_bgt0d:26857 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
ubi_bgt0d     D dd37bf94     0 26857      2
       dd37bfcc 00000086 f8e17cea dd37bf94 00000046 00000000 00000000 f5c62430
       f5c62430 f5c62590 c2a09c80 f6cbd498 dd8e9cbc 00000296 dd37bfb0 00000296
       dd8e9cb8 dd8e9cbc dd37bfcc c0119774 00000000 00000000 c0132e89 f6961560
Call Trace:
 [<f8e17cea>] ? ubi_thread+0x0/0x127 [ubi]
 [<c0119774>] ? complete+0x43/0x4b
 [<c0132e89>] ? kthread+0x0/0x5b
 [<f8e17cea>] ? ubi_thread+0x0/0x127 [ubi]
 [<c0132eae>] kthread+0x25/0x5b
 [<c0132e89>] ? kthread+0x0/0x5b
 [<c0104953>] kernel_thread_helper+0x7/0x14
 =======================

So start it, and go sleep inside it, instead of creating it and never
start.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>