mfd: dm355evm_msp: Return directly after a failed platform_device_alloc() in add_child()

The platform_device_put() function was called in one case by the
add_child() function during error handling even if the passed
variable "pdev" contained a null pointer.
Return directly in this case.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>