spi: bcm2835: Increase the CS limit to 24

Increase the maximum number of CS lines to 24, and ensure this limit is
not exceeded.

See: https://github.com/raspberrypi/linux/pull/4281



Suggested-by: Joe Burmeister <joe.burmeister@devtank.co.uk>
Signed-off-by: Phil Elwell <phil@raspberrypi.com>