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.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Reported-by: Sven Neumann <s.neumann@raumfeld.com>
Reported-by: Andreas Fenkart <andreas.fenkart@streamunlimited.com>
Tested-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Tested-by: Andreas Fenkart <andreas.fenkart@streamunlimited.com>
Signed-off-by: David S. Miller <davem@davemloft.net>