mfd: max8998: Fix potential NULL pointer dereference

if 'max8998_i2c_parse_dt_pdata() fails (when out of memory), a NULL
pointer dereference will occur in the error handling code.

Return directly instead.

Fixes: ee999fb3("mfd: max8998: Add support for Device Tree")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>