staging: r8188eu: convert three functions to bool

staging: r8188eu: convert three functions to bool

The functions

is_client_associated_to_ap()
is_client_associated_to_ibss()
is_IBSS_empty()

return boolean values. Convert their return type to bool and replace
_FAIL, which is defined as 0, with false. Another step to get rid of
_SUCCESS / _FAIL.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20221105093916.8255-1-straube.linux@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>