net/mlx5: E-Switch, UBSAN fix undefined behavior in mlx5_eswitch_mode

With debug kernel UBSAN detects the following issue, which might happen
when eswitch instance is not created, fix this by testing the eswitch
pointer before returning the eswitch mode, if not set return mode =
SRIOV_NONE.

[   32.528951] UBSAN: Undefined behaviour in drivers/net/ethernet/mellanox/mlx5/core/eswitch.c:2219:12
[   32.528951] member access within null pointer of type 'struct mlx5_eswitch'
[   32.528951] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.0-rc3-dirty #181
[   32.528951] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
[   32.528951] Call Trace:
[   32.528951]  dump_stack+0xc7/0x13b
[   32.528951]  ? show_regs_print_info+0x5/0x5
[   32.528951]  ? __pm_runtime_use_autosuspend+0x140/0x140
[   32.528951]  ubsan_epilogue+0x9/0x49
[   32.528951]  ubsan_type_mismatch_common+0x1f9/0x2c0
[   32.528951]  ? ucs2_as_utf8+0x310/0x310
[   32.528951]  ? device_initialize+0x229/0x2e0
[   32.528951]  __ubsan_handle_type_mismatch+0x9f/0xc9
[   32.528951]  ? __ubsan_handle_divrem_overflow+0x19b/0x19b
[   32.578008]  ? ib_device_get_by_index+0xf0/0xf0
[   32.578008]  mlx5_eswitch_mode+0x30/0x40
[   32.578008]  mlx5_ib_add+0x1e0/0x4a0

Fixes: 57cbd893 ("net/mlx5: E-Switch, Move representors definition to a global scope")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>