SAMD -Wtype-limits warnings in set_clock_disable (si5351.cpp:972, 984) #109

CI on the SAMD board (arduino:samd:mkrzero) reports two -Wtype-limits warnings in set_clock_disable():

src/si5351.cpp:972:10: warning: comparison is always true due to limited range of data type [-Wtype-limits]
src/si5351.cpp:984:10: warning: comparison is always true due to limited range of data type [-Wtype-limits]

Both lines test clk >= SI5351_CLK0. SI5351_CLK0 is 0, and on that toolchain the enum is unsigned, so the lower-bound check can never fail. These warnings existed before #108 and don't change behavior.

Suggested fix: drop the redundant clk >= SI5351_CLK0 comparisons, or compare as a signed or explicitly cast value. Then confirm the SAMD build is warning-free.

Found during the independent review of #108.