block: don't allow nr_pending to go negative

nr_pending can go negative if we attempt to decrement it without matching
increment call earlier. If nr_pending goes negative, LLD's runtime suspend
might race with the ongoing request. This change allows decrementing
nr_pending only if it is non-zero.

Change-Id: I5f1e93ab0e0f950307e2e3c4f95c7cb01e83ffdd
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>