mmc: dw_mmc: convert the variable type of irq

Even though platform_get_irq returns error, 'host->irq'
always has an unsigned value. Less-than-zero comparison
of an unsigned value is never true. Type of 'unsigned int'
will be changed for 'int'.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>