i2o: core: implemented i2o_dev_request_mode.

Added the following functions:
	* i2o_dev_request_mode
	* i2o_dev_current_mode
	* i2o_dev_set_mode

i2o_dev_request_mode is an internal function which can be used by, for
example i2o_dev_xfer, the core module to change the current mode of the
I2O device. A change is not guaranteed. It will try to aquire the device
lock, but if it fails to aquire it a negative error code will be
returned.

i2o_dev_current_mode is an inline helper function located in
include/linux/interio/i2o.h. The main purpose of this function is to
return the currently set mode of a given I2O device. Before the mode is
returned, it will be checked against a mask of all currently available
modes for that device.

i2o_dev_set_mode can, as the name implies, be used to change the mode of
a given I2O device. Before the mode is applied to the device, it is
checked against a mask of all available modes for that device. The
result of that check will be set and returned.

Signed-off-by: Michel Megens <dev@michelmegens.net>