msm: ipc: Possible memory corruption due to Sign Conversion

msm_ipc_router_skb_to_buf() takes an unsigned argument and assigns the
same to a signed local variable. This might cause issues when the value
of the argument is too high.

Change the datatype of the local variable to unsigned.

CRs-Fixed: 550606
Change-Id: I257a095681dd82fba05367fd6faf25820e95c719
Signed-off-by: Zaheerulla Meer <zmeer@codeaurora.org>