msm: pmic: Fix transmit buffer overflow

pmic_buf_reserve() API should have kept the accounting of
length in the pmic_buf structure instead of pmic_rpc_req_reply()
in-order to avoid the overflow for the cases in the
the pmic_rpc_set_struct() API where size is more than
212 bytes.

CRs-Fixed: 294864
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>