mac_pton: Don't access memory over expected length

The strlen() may go too far when estimating the length of
the given string. In some cases it may go over the boundary
and crash the system which is the case according to the commit
13a55372 ("ARM: orion5x: Revert commit 4904dbda

.").

Rectify this by switching to strnlen() for the expected
maximum length of the string.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221108141108.62974-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>