If you look at the MPU9250 driver, you will see that it is trying to use the
PX4_SPI_BUS_SENSORS bus as the SPI bus. This bus is set in the board_config.h file for the board (remember there is a board_config.h for each board). In board_config.h you will probably see something like this:
#define PX4_SPI_BUS_SENSORS 1
#define PX4_SPI_BUS_RAMTRON 2
#define PX4_SPI_BUS_BARO PX4_SPI_BUS_RAMTRON
If you change the PX4_SPI_BUS_SENSORS bus to be 2, then in px4fmu_spi.c then you will also have to change the SPI select function from stm32_spi1select() to be stm32_spi2select().
But be careful. If you change the MPU9250 to be on another SPI bus, are you changing all of your sensors for your new board to be on this new bus. The one assumption that is made in most of the Px4 sensor drivers is that the onboard sensors are all on the same