ntb_hw_switchtec: Fix shift-out-of-bounds in switchtec_ntb_mw_set_trans

There is a kernel API ntb_mw_clear_trans() would pass 0 to both addr and
size. This would make xlate_pos negative.

[   23.734156] switchtec switchtec0: MW 0: part 0 addr 0x0000000000000000 size 0x0000000000000000
[   23.734158] ================================================================================
[   23.734172] UBSAN: shift-out-of-bounds in drivers/ntb/hw/mscc/ntb_hw_switchtec.c:293:7
[   23.734418] shift exponent -1 is negative

Ensuring xlate_pos is a positive or zero before BIT.

Fixes: 1e2fd202 ("ntb_hw_switchtec: Check for alignment of the buffer in mw_set_trans()")
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>