i2c: i2c-sh_mobile irq rollback fix

Update the i2c-sh_mobile driver to properly free
interrupts. The function sh_mobile_i2c_hook_irqs()
is fixed so module both unload and load are working
as expected.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>