i2c: efm32: Avoid unnecessary check in efm32_i2c_probe()

The function efm32_i2c_probe() is only called with an
openfirmware platform device.Therefore there is no need
to check that it has an openfirmware node.

Signed-off-by: Shengju Zhang <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>