bnxt_en: Return error when allocating zero size context memory.

bnxt_alloc_ctx_pg_tbls() should return error when the memory size of the
context memory to set up is zero.  By returning success (0), the caller
may proceed normally and may crash later when it tries to set up the
memory.

Fixes: 08fe9d18 ("bnxt_en: Add Level 2 context memory paging support.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>