hw/sd.c: fix sd_set_cb() crash when bdrv == NULL

sd_set_cb() calls bdrv_is_read_only() and bdrv_is_inserted() even if
no block driver is associated with the card reader.

This patch fixes the issues by not setting the irq in this case, this
fixes ARM versatile crash.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
