Staging: sb105x: Fix a possible null dereference

This patch fixes the following error in sb_pc_mp.c-

drivers/staging/sb105x/sb_pci_mp.c:546 mp_startup() error: we previously assumed 'info->tty' could be null (see line 525)

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>