net: xilinx: axi_emac: Typecast flush_cache arguments

flush_cache() arguments are not type casted to take care of 64 bit
systems. Use phys_addr_t to type cast for it to work properly for 32 bit
and 64 bit systems.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>