m68k: atari: Update Kconfig.bus help text

Add NetUSBee to users of ATARI_ROM_ISA option (thanks Geert for
spotting this in randconfig builds).

Add note to ATARI_USB option to alert user to enable ATARI_ROM_ISA
if they want support for the NetUSBee USB driver.

Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>