[ARM] msm: smd: Force the half_channel struct aligmnent to 4

Forcing the alignment prevents gcc from generating byte reads for word
member variables. Lack of this caused issues when the app processor
modified struct members and the modem saw a partial word write.

Signed-off-by: Dima Zavin <dima@android.com>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>