b43: Add slot count compiletime assertion

This adds a compiletime assertion for a recently introduced
assumption on the slot counts.
The tx header cache handling code assumes that the TX slot count
can be divided evenly by the number of TX slots per frame.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>