regmap: fix a NULL pointer dereference in __regmap_init

__regmap_init() may receive a NULL `struct regmap_bus *bus' pointer,
for example, from snd_hdac_regmap_init(), and it make sure that it
does not NULL deference `bus`, except around ->max_raw_read and
->max_raw_write initialisation. Add missing check.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>