msm: cleanup: struct gsi_wdi2_channel_scratch2_reg

Remove __packed from the usage of union gsi_wdi3_channel_scratch2_reg.
__packed is only needed in the union inclusion.

Bug: 152368391
Change-Id: I7a5df200b4cbf12b5bcbe13497987b696eec76b8
Signed-off-by: Wilson Sung <wilsonsung@google.com>