uio: Fix return value of poll

The return type of __poll_t can't be negative,
-EIO will be translate to __poll_t,and will return to caller.

Fixes: beafc54c ("UIO: Add the User IO core code")
Signed-off-by: Angus Chen <angus.chen@jaguarmicro.com>
Link: https://lore.kernel.org/r/20241126124259.1367-1-angus.chen@jaguarmicro.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>