msm: camera: Fix incorrect update in EEPROM packet parser

Fix the incorrect update to the cmd_buf pointer and
also the size of processed bytes to ensure that
PWR_DOWN command can be parsed correctly. This ensures
proper power down of EEPROM.

Change-Id: I6f54d8039755052d0620d441b9280c3e4ade8c32
Signed-off-by: Ravi Kishore Tanuku <rktanuku@codeaurora.org>