msm: adsprpc: Use unsigned integer for length values

As the length datatype is signed, an attacker can both overflow
the calculation or supply a negative number to trick the check
into returning an chosen chunk. This can have undesired
consequences. Always use unsigned integer types for length
values.

Change-Id: Ifde2f0d35129014b976507f7723a319c53fabddf
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>