[PATCHv5 03/10] HSI: export method to (un)register clients

Expose method for registering and unregistering HSI clients, so that client drivers can register other client drivers. This is useful for HSI drivers, which want to use the functionality of other HSI drivers. For example the N900 modem driver can load HSI drivers for mcsaab protocol and speech

[PATCHv5 03/10] HSI: export method to (un)register clients

Expose method for registering and unregistering HSI clients, so that client drivers can register other client drivers. This is useful for HSI drivers, which want to use the functionality of other HSI drivers. For example the N900 modem driver can load HSI drivers for mcsaab protocol and speech