mgmt/osdp: Fix off-by-one in buf len checks

Initially, the command/reply ID byte was not part of the data length
macros. But later, when it was changed to include it, the buffer length
checks was not adjusted. Due to this, we were not using the last byte in
the buffer. Fix this issue by correcting the condition.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>