bluetooth: Refactor POLLHUP handling

bluetooth: Refactor POLLHUP handling

The code in the "io_fail" section was only used for HUP handling, but
there were jumps to there also from places where reading or writing
failed, because the read/write failure could have been caused by HUP.
This patch simplifies things by checking for HUP condition before
trying to read or write. Now if reading or writing fails, we will
jump to "fail" directly instead of going via the "io_fail" label. As
a result, the "io_fail" label isn't needed any more.