Fix i2c_smbus_read_i2c_block_data return value check

[illiav-j]

Fixes the error when trying to build on the latest OpenBMC:

| …/src/vr_update_mps.cpp: In member function 'virtual bool vr_update_mps::crcCheckSum()':
| …/src/vr_update_mps.cpp:32:15: error: ordered comparison of pointer with integer zero ('uint8_t*' {aka 'unsigned char*'} and 'int')
|    32 |     if (rdata < SUCCESS)
| …/src/vr_update_mps.cpp: In member function 'virtual bool vr_update_mps::isUpdatable()':
| …/src/vr_update_mps.cpp:83:14: error: ordered comparison of pointer with integer zero ('uint8_t*' {aka 'unsigned char*'} and 'int')
|    83 |     if(rdata >= SUCCESS)
| …/src/vr_update_mps.cpp:95:14: error: ordered comparison of pointer with integer zero ('uint8_t*' {aka 'unsigned char*'} and 'int')
|    95 |     if(rdata >= SUCCESS)

This change has not been tested.