bnxt_en: Fix comparison of u16 sw_id against negative value.

Assign the return value from bitmap_find_free_region() to an integer
variable and check for negative error codes first, before assigning
the bit ID to the unsigned sw_id field.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>