bnxt_en: Return error if bnxt_alloc_ctx_mem() fails.

The current code ignores the return value from
bnxt_hwrm_func_backing_store_cfg(), causing the driver to proceed in
the init path even when this vital firmware call has failed.  Fix it
by propagating the error code to the caller.

Fixes: 1b9394e5 ("bnxt_en: Configure context memory on new devices.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>