imx_fec: Use ENET_FTRL to determine truncation length

imx_fec: Use ENET_FTRL to determine truncation length

Frame truncation length, TRUNC_FL, is determined by the contents of
ENET_FTRL register, so convert the code to use it instead of a
hardcoded constant.

To avoid the case where TRUNC_FL is greater that ENET_MAX_FRAME_SIZE,
increase the value of the latter to its theoretical maximum of 16K.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: yurovsky@gmail.com
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>