signal: fix typo in dequeue_synchronous_signal()

s/postive/positive/

Signed-off-by: Pavel Machek (CIP) <pavel@denx.de>
Link: https://lore.kernel.org/r/20200724090531.GA14409@amd


[christian.brauner@ubuntu.com: tweak commit message]
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>