lavfi/gradfun: reduce up limit for threshold.

This will prevent an overflow in the SSSE3 and MMX filter_line code:
delta is expressed as an u16 being shifted by 2 to the left. If it
overflows, having a strength not above 64 will make sure that m is set
to 0 (making the m*m*delta >> 14 expression void).

A value above 64 should not make any sense unless gradfun is used as a
blur filter.
