mmc: core: Fix NULL pointer dereference

mmc_signal_sdio_irq() should only be called when the function
driver registers for IRQ notification. In case, if any host
driver recieves a spurious interrupt and call mmc_signal_sdio_irq(),
ignore waking up uninitialized sdio irq thread.

CRs-Fixed: 472609
Change-Id: I831717fe80918538a18cc1f2300fe7fe2817c70f
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>