mgmt/osdp: Skip all incoming bytes till start of new packet

All OSDP packets must start with a mark byte (0xFF) followed by a Start
of Message byte (0x53). Skip all bytes received until such a sequence is
seen to to avoid waking up the refresh thread needlessly.

Also, refactor osdp_uart_isr() to reduce number of local variables.

Fixes: #28168 Coverity-ID: 214218

Signed-off-by: Siddharth Chandrasekaran <siddharth@embedjournal.com>