omap: iovmm: Add missing mutex_unlock

I was using Coccinelle with the mutex_unlock semantic patch, and it
unconvered this problem. It appears to be a valid missing unlock issue.
This change should correct it by moving the unlock below the label.

This patch is against the mainline kernel.

Cc: Julia Lawall <julia@diku.dk>
Cc: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Daniel Walker <dwalker@fifo99.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>