net/macb: replace macb_count_tx_descriptors() by DIV_ROUND_UP()

macb_count_tx_descriptors() repeats the generic macro DIV_ROUND_UP(). The patch
does a replacement.

There is no functional change.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>