ipmi: fix ipmb_poll()'s return type

ipmb_poll() is defined as returning 'unsigned int' but the
.poll method is declared as returning '__poll_t', a bitwise type.

Fix this by using the proper return type and using the EPOLL
constants instead of the POLL ones, as required for __poll_t.

CC: Corey Minyard <minyard@acm.org>
CC: openipmi-developer@lists.sourceforge.net
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Message-Id: <20191120000741.30657-1-luc.vanoostenryck@gmail.com>
Reviewed-by: Asmaa Mnebhi <asmaa@mellanox.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>