Bluetooth: btmtksdio: Prevent enabling interrupts after IRQ handler removal

Ensure interrupts are not re-enabled when the IRQ handler has already been
removed. This prevents unexpected IRQ handler execution due to stale or
unhandled interrupts.

Modify btmtksdio_txrx_work to check if bdev->func->irq_handler exists
before calling sdio_writel to enable interrupts.

Co-developed-by: Pedro Tsai <pedro.tsai@mediatek.com>
Signed-off-by: Pedro Tsai <pedro.tsai@mediatek.com>
Co-developed-by: Felix Freimann <felix.freimann@mediatek.com>
Signed-off-by: Felix Freimann <felix.freimann@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>