scmi: Fix token bitmask

By the SCMI specification the message sequence number field is 10-bit long.
Fix accordingly the used bitmask.

Fixes: d220b1cf02ce ("scmi: Extend and refactor SCMI shmem support")
Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47418
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>