generic: fix wrong PTR_ERR use in of net mac address ascii patch

generic: fix wrong PTR_ERR use in of net mac address ascii patch

nvmem_cell_read return a pointer error when an error occurs. Currently
we convert the pointer error to an int while the rest of the function
return a void* and expcet an error pointer. Fix this PTR_ERR msuse
fixing compilation warning.

Fixes the following compilation warning:
net/ethernet/eth.c: In function 'nvmem_cell_get_mac_address':
net/ethernet/eth.c:547:24: warning: returning 'long int' from a function with return type 'void *' makes pointer from integer without a cast [-Wint-conversion]
  547 |                 return PTR_ERR(mac);
      |                        ^~~~~~~~~~~~
net/ethernet/eth.c: In function 'nvmem_cell_get_mac_address_ascii':
net/ethernet/eth.c:564:24: warning: returning 'long int' from a function with return type 'void *' makes pointer from integer without a cast [-Wint-conversion]
  564 |                 return PTR_ERR(mac_ascii);
      |                        ^~~~~~~~~~~~~~~~~~

Fixes: 7b863af180da ("kernel: move mac-address-ascii patches to generic")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>