i2c: mxc: add a condition in case the parameter is NULL

This could avoid executing the code that only applies to i.MX platforms.

The bus_i2c_init() is called before relocation and will assgin value
to a static variable. If U-Boot is then still running in a flash
device, it's theoretically not allowed to write data to flash without
an erasing operation. For i.MX platforms, the U-Boot is always running
in DDR.

Actually it causes asynchronous error when the ARM64 system error
report is enabled and the flash write protect is set.

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: Heiko Schocher <hs@denx.de>