Mark a few functions static inline

Mark a few functions static inline

The function prototypes that are defined in the header most likely has
to be marked as static inline. This helps avoiding the compiler warnings:

  include/mux.h:120:14: warning: no previous prototype for ‘mux_control_states’ [-Wmissing-prototypes]
  include/mux.h:125:18: warning: no previous prototype for ‘mux_control_select’ [-Wmissing-prototypes]
  include/mux.h:133:5: warning: no previous prototype for ‘mux_control_deselect’ [-Wmissing-prototypes]
  include/mux.h:138:21: warning: no previous prototype for ‘mux_control_get’ [-Wmissing-prototypes]
  include/mux.h:143:6: warning: no previous prototype for ‘mux_control_put’ [-Wmissing-prototypes]
  include/mux.h:147:21: warning: no previous prototype for ‘devm_mux_control_get’ [-Wmissing-prototypes]
  include/mux.h:153:5: warning: no previous prototype for ‘dm_mux_init’ [-Wmissing-prototype ]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>