firmware: microchip: fix wrong sizeof argument

response_msg is a pointer to an unsigned int (u32). So passing just
response_msg to sizeof would not print the size of the variable. To get
the size of response_msg we need to pass it as a pointer variable.

Fixes: ec5b0f11

 ("firmware: microchip: add PolarFire SoC Auto Update support")
Signed-off-by: Samasth Norway Ananda <samasth.norway.ananda@oracle.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>