net: zynq_gem: Use ulong instead of u32 data type

flush_dcache_range() expects unsigned long in the arguments. Here u32
variable is unable to hold the higher address value when ddr mapped
to higher addresses & flushing lower address dchache range instead
which is unmapped causing to crash.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>