mcumgr: img_mgmt_client: Fix Image upload max data length

MCUmgr client upload max data payload length was missing
Transport layer CRC + 16bit lenght. Full net buf packet was
possible to send but receiver side it was blocked because there
was not space for calculate CRC and length.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>