bus: bt1-apb: Use PTR_ERR_OR_ZERO to return from request-regs method

Indeed it's more optimal to use the PTR_ERR_OR_ZERO() macro there instead
of having two return points.

Link: https://lore.kernel.org/r/20200528145050.5203-3-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Olof Johansson <olof@lixom.net>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: soc@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>