hw/net/imx_fec: write TGSR and TCSR3 in imx_enet_write()

hw/net/imx_fec: write TGSR and TCSR3 in imx_enet_write()

The current code causes clang static code analyzer generate warning:
hw/net/imx_fec.c:858:9: warning: Value stored to 'value' is never read
        value = value & 0x0000000f;
        ^       ~~~~~~~~~~~~~~~~~~
hw/net/imx_fec.c:864:9: warning: Value stored to 'value' is never read
        value = value & 0x000000fd;
        ^       ~~~~~~~~~~~~~~~~~~

According to the definition of the function, the two “value” assignments
 should be written to registers.

Reported-by: Euler Robot <[email protected]>
Signed-off-by: Chen Qun <[email protected]>
Message-id: 20200313123242[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>