[media] msi2500: Delete an unnecessary check in msi2500_set_usb_adc()

This issue was detected by using the Coccinelle software.

Return the value from a call of the msi2500_ctrl_msg() function
without using an extra check for the variable "ret" at the end.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>