stmmac: platform: adjust messages and move to dev level

This patch amends the error and warning messages across the platform driver.
It includes the following changes:
 - append \n to the end of messages
 - change pr_* macros to dev_*

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