net: zynq: Free allocated buffers in case of error

Driver probe function is called again and again in case of error.
Malloc space is getting full which is is reported by:
 Insufficient RAM for page table: 0x15000 > 0x14000.
 Please increase the size in get_page_table_size()
 ### ERROR ### Please RESET the board ###

The patch is freeing allocated buffers on error path to avoid panic.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>