Char: cyclades, allow DEBUG_SHIRQ

Test if base addr is non-null in ISR to prove the card has been correctly
initialized.  This is needed for DEBUG_SHIRQ for example.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>