HID: hid-steam: Clean up locking

This cleans up the locking logic so that the spinlock is consistently used for
access to a small handful of struct variables, and the mutex is exclusively and
consistently used for ensuring that mutliple threads aren't trying to
send/receive reports at the same time. Previously, only some report
transactions were guarded by this mutex, potentially breaking atomicity. The
mutex has been renamed to reflect this usage.

Signed-off-by: Vicki Pfau <vi@endrift.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>