haven: doorbell: Remove locks around read/send

Hypercalls are guaranteed to complete from perspective of kernel and
serialization of hypercalls across cores is not required. Thus, in order
to support sending/receiving doorbells in interrupt contexts, remove
mutex locking in the respective functions.

Change-Id: I01993215ea998de935b868144b355fae937bf138
Signed-off-by: Elliot Berman <eberman@codeaurora.org>