mfd: vexpress: fix error handling vexpress_syscfg_regmap_init()

This function should be returning an ERR_PTR() on failure instead of
NULL.  Also there is a use after free bug if regmap_init() fails because
we free "func" and then dereference doing the return.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>