drivers: net: ethernet: ti: davinci_emac: fix usage of cpdma_check_free_tx_desc()

Fix which was done in the following commit in cpsw driver has
to be taken forward to davinci emac driver as well.

commit d35162f8
Author: Daniel Mack <zonque@gmail.com>
Date:   Tue Mar 12 06:31:19 2013 +0000

    net: ethernet: cpsw: fix usage of cpdma_check_free_tx_desc()

    Commit fae50823 ("net: ethernet: davinci_cpdma: Add boundary for rx
    and tx descriptors") introduced a function to check the current
    allocation state of tx packets. The return value is taken into account
    to stop the netqork queue on the adapter in case there are no free
    slots.

    However, cpdma_check_free_tx_desc() returns 'true' if there is room in
    the bitmap, not 'false', so the usage of the function is wrong.

Reported-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Tested-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>