char: i.MX: Simplify imx_update()

char: i.MX: Simplify imx_update()

Code of imx_update() is slightly confusing since the "flags" variable
doesn't really corespond to anything in real hardware and server as a
kitchensink accumulating events normally reported via USR1 and USR2
registers.

Change the code to explicitly evaluate state of interrupts reported
via USR1 and USR2 against corresponding masking bits and use the to
detemine if IRQ line should be asserted or not.

NOTE: Check for UTS1_TXEMPTY being set has been dropped for two
reasons:

    1. Emulation code implements a single character FIFO, so this flag
       will always be set since characters are trasmitted as a part of
       the code emulating "push" into the FIFO

    2. imx_update() is really just a function doing ORing and maksing
       of reported events, so checking for UTS1_TXEMPTY should happen,
       if it's ever really needed should probably happen outside of
       it.

Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: Bill Paul <wpaul@windriver.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Message-id: 20180315191141.6789-1-andrew.smirnov@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>