regmap: fix kernel hang on regmap_bulk_write with zero val_count.

regmap: fix kernel hang on regmap_bulk_write with zero val_count.

If val_count is zero we return -EINVAL with map->lock_arg locked, which
will deadlock the kernel next time we try to acquire this lock.

In 3.12, this was introduced by a0b8d8d906d267987d507138003048c5fdf774
("regmap: fix possible ZERO_SIZE_PTR pointer dereferencing error.")
which improperly back-ported d6b41cb06044a7d895db82bdd54f6e4219970510.

This issue was found during review of Ubuntu Trusty 3.13.0-40.68 kernel to
prepare Ksplice rebootless updates.

Fixes: f5942dd ("regmap: fix possible ZERO_SIZE_PTR pointer dereferencing error.")
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>