regulator: da9063: fix assignment of da9063_reg_matches to NULL

cppcheck detected an incorrect assignment:

 drivers/regulator/da9063-regulator.c:711]: (warning) Assignment
 of function parameter has no effect outside the function

the original code didn't do anything, instead, *da9063_reg_matches
needs to be set to NULL.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@linaro.org>