Fix NPU driver inference poll callback return type

The poll file operation function pointer is expected to return the
__poll_t type but the currently implemented poll inference function
returns an unsigned int. This breaks the type safety and causes Sparse
to generate a warning.

To resolve this, the return type has been changed to __poll_t and the
function now uses with EPOLL constants that share the same type.

Change-Id: I41dadb758c7c4b42b431d96a94c6b2cc9f960013
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>