greybus: uart: properly calculate max buffer size

We forgot to count the size of the uart send data message header when
calculating the maximum size of the buffer that the uart driver could
send in one chunk.

This fixes the math and makes the variable a size_t to match the return
value of the call to gb_operation_get_payload_size_max();

Reported-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Tested-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>