fsl/fman: Check for null pointer after calling devm_ioremap

As the possible failure of the allocation, the devm_ioremap() may return
NULL pointer.
Take tgec_initialization() as an example.
If allocation fails, the params->base_addr will be NULL pointer and will
be assigned to tgec->regs in tgec_config().
Then it will cause the dereference of NULL pointer in set_mac_address(),
which is called by tgec_init().
Therefore, it should be better to add the sanity check after the calling
of the devm_ioremap().

Fixes: 39339616 ("fsl/fman: Add FMan MAC driver")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>