lib: qmi_encdec: Ignore unknown optional TLV

Currently, Kernel QMI encode/decode library returns an error upon
receiving an unknown optional TLV. Any optional TLV in a received
QMI message can be ignored and the rest of the message can be decoded.

Add support to ignore the unknown optional TLV and decode rest of
the message.

Change-Id: I8bb509879422c9193c00ed4ef847b0da1d642e33
Signed-off-by: Atish Kumar Patra <apatra@codeaurora.org>