Bluetooth: Increase default TX stack size

The current stack consumption with the H4 driver on qemu_x86 is as
follows with a change from 256 to 512:

	usage 396 / 512 (77 %)

Increase the default for this configuration, as well as other similar
configurations. Set the fallback default to a higher value as well.

Fixes #12429

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>