mailbox: mailbox-test: make data_ready a per-instance variable

mailbox: mailbox-test: make data_ready a per-instance variable

While not the default case, multiple tests can be run simultaneously.
Then, data_ready being a global variable will be overwritten and the
per-instance lock will not help. Turn the global variable into a
per-instance one to avoid this problem.

Fixes: e339c80af95e ("mailbox: mailbox-test: don't rely on rx_buffer content to signal data ready")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>