[Usbb2k-api-dev] Experimental support for the P4K in yealink.ko

[yealink]

Hi all,

Based on Steves reverse engineering work I've added experimental support
for the P4K phone.

The module has an additional parameter "default_model" that will pre
configure the phone to either a P1K or a P4K.

insmod yealink.ko default_model=P1K
insmod yealink.ko default_model=P4K

Also the model is runtime configurable by using /sys/./model on a phone
by phone basis.

I.ve also added support for BACKLIGHT and SPEAKER via the show-, hide-,
and get_icon interface.

I don't have the hardware myself so I would have to rely on some of you
guys to test it.

have fun,
Henk