hal: Update VoIP output calculations for stereo channels

HAL assumes VoIP output supports mono configuration only.
Buffer size, period calculations do not account for channels.
At present, if VoIP is opened with stereo channels, buffer
size returned to framework is incorrect.
Update buffer size and period size calculations for VoIP
output for stereo channel configuration.

Change-Id: Ib30dde389c1a93fcfe8b0f68d50f2f2327f20489