[FIRRTL] Fix width of attribute in MuxPadSel canonicalizations.

[dtzSiFive]

Detected by -DMLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS=ON.

cc #7047.

[dtzSiFive]

/home/will/src/sifive/circt/test/Dialect/FIRRTL/canonicalization.mlir:628:9: error: firrtl.constant attribute bitwidth doesn't match return type
  %16 = firrtl.mux (%val0, %val1, %val2) : (!firrtl.uint<0>, !firrtl.uint<1>, !firrtl.uint<1>) -> !firrtl.uint<1>
  ¦ ¦ ¦ ^
/home/will/src/sifive/circt/test/Dialect/FIRRTL/canonicalization.mlir:628:9: note: see current operation: %9 = "firrtl.constant"() {value = 0 : ui32} : () -> !firrtl.uint<1>
LLVM ERROR: IR failed to verify after pattern application

[uenoku]

LGTM!