kernel: Change k_poll_signal api

k_poll_signal was being used by both, struct and function. Besides
this being extremely error prone it is also a MISRA-C violation.
Changing the function to contain a verb, since it performs an action
and the struct will be a noun. This pattern must be formalized and
followed and across the project.

MISRA-C rules 5.7 and 5.9

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>