Fix interrupt clear in pl011 uart receive function

[jchandra]

Summary

Clear interrupt status register before reading the input char from
the FIFO. In the current code the ICR is cleared after the read and
this will leave a window a new char coming into the FIFO after the
read is done but before ICR is updated will not cause an interrupt.

This fixes the issue by which the serial port input on QEMU freezes
when using the emulated pl011 serial port.

[emaste]

This was committed in rS309276. Thanks so much for fixing it!

For future uploads can you please include more / full context with something like git diff -U999999 other-branch (or see https://wiki.freebsd.org/Phabricator for svn details).