axonram: Return directly after a failed kzalloc() in axon_ram_probe()

* Return directly after a call of the function "kzalloc" failed
  at the beginning.

* Delete a repeated check for the local variable "bank"
  which became unnecessary with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>