In a few places, sdio_uart_irq() is called directly instead of waiting
for the actual interrupt to be raised and the SDIO IRQ thread scheduled
in order to reduce latency.  However, some interaction with the tty core
may end up calling us back (serial echo, flow control, etc.) creating
two issues:

 - the host lock gets claimed twice from the same thread causing a
   deadlock;

 - the same direct calls to sdio_uart_irq() may be performed causing
   unexpected reentrancy into the IRQ handler.

This patch handles both of those issues.

Signed-off-by: Nicolas Pitre <npitre@mvista.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>