USB: misc: legousbtower: semaphore to mutex

The dev->sem conforms to mutex style usage. This patch converts it to use
the struct mutex type, and new API.

There is also a small style fix around this comment,

/* unlock here as tower_delete frees dev */

Where I broke the line up to meet the 80 char limit.

Signed-off-by: Daniel Walker <dwalker@mvista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>