ASoC: compr: wake up drain only if it's the last buffer

There's a corner case where there're pause and resume operations in the
middle of last full buffer drain. Waking up drain without checking
against the real byte available violates the assumption only one
partial buffer in the last write.

The solution is to just resume the write pipe if there's more than one
fragment pending.

CRs-Fixed: 2536883
Change-Id: Id24c63bad774ec916942b8f94ddaa396d2560517
Signed-off-by: Weiyin Jiang <wjiang@codeaurora.org>