serial: imx: clean up imx_poll_get_char()
serial: imx: clean up imx_poll_get_char()

Looking at the get_poll_char() function of the 8250.c serial driver,
we learn:

* poll_get_char() doesn't have to save/disable/restore the interrupt
  registers. No interrupt handling is needed in this function at all.
  Remove it.

* Don't block in case there is no data available. So instead blocking
  in the do {} while loop, just return with NO_POLL_CHAR, immediately .

Additionally, while the i.MX6 register URXD[7-0] contain the RX_DATA,
the upper bits of this register (URXD[15-10]) might contain some
control flags. To ensure that these are not returned with the data
read, just mask out URXD[7-0].

These changes fix the 'hang' working with kdb:

$ echo ttymxc3 > /sys/module/kgdboc/parameters/kgdboc
$ echo g >/proc/sysrq-trigger
[0]kdb> help
...
<hang>

Signed-off-by: Dirk Behme <65191830245723325aed938dddb6d81810e13fc4@de.bosch.com>
Signed-off-by: Daniel Thompson <4421c302a8f2019ff02452a16ddbdd37770f814c@linaro.org>
Cc: Jiri Slaby <dfbf39eb5bf303a3db5454d815cb8c8888cbadf9@suse.cz>
Cc: f97bd3735ebb06ec9fde555afdf685a9966cf521@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <4645f7897fd33786a2ee1264d590b3c400559d85@linuxfoundation.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs