ntb: ntb_hw_switchtec: Fix shift-out-of-bounds for 0 mw lut

In the Linux kernel, the following vulnerability has been resolved:

ntb: ntb_hw_switchtec: Fix shift-out-of-bounds for 0 mw lut

Number of MW LUTs depends on NTB configuration and can be set to zero,

in such scenario rounddown_pow_of_two will cause undefined behaviour and

should not be performed.

This patch ensures that rounddown_pow_of_two is called on valid value.