asm-generic: make parameter types consistent in _unaligned_be48()

There is a convention to use internal kernel types, so replace __u8 by u8.

Link: https://lkml.kernel.org/r/20220830172713.43686-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>