net/mlx5: Protect from command bit overflow

The system with CONFIG_UBSAN enabled on produces the following error
during driver initialization. The reason to it that max_reg_cmds can be
larger enough to cause to "1 << max_reg_cmds" overflow the unsigned long.

================================================================================
UBSAN: Undefined behaviour in drivers/net/ethernet/mellanox/mlx5/core/cmd.c:1805:42
signed integer overflow:
-2147483648 - 1 cannot be represented in type 'int'
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.15.0-rc2-00032-g06cda2358d9b-dirty #724
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
Call Trace:
 dump_stack+0xe9/0x18f
 ? dma_virt_alloc+0x81/0x81
 ubsan_epilogue+0xe/0x4e
 handle_overflow+0x187/0x20c
 mlx5_cmd_init+0x73a/0x12b0
 mlx5_load_one+0x1c3d/0x1d30
 init_one+0xd02/0xf10
 pci_device_probe+0x26c/0x3b0
 driver_probe_device+0x622/0xb40
 __driver_attach+0x175/0x1b0
 bus_for_each_dev+0xef/0x190
 bus_add_driver+0x2db/0x490
 driver_register+0x16b/0x1e0
 __pci_register_driver+0x177/0x1b0
 init+0x6d/0x92
 do_one_initcall+0x15b/0x270
 kernel_init_freeable+0x2d8/0x3d0
 kernel_init+0x14/0x190
 ret_from_fork+0x24/0x30
================================================================================

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>