iio: freq: admv1014: Fix warning about dubious x & !y and improve readability

The warning comes from __BF_FIELD_CHECK()
specifically

BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?		\
		 ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
		 _pfx "value too large for the field"); \

The code was using !(enum value) which is not particularly easy to follow
so replace that with explicit matching and use of ? 0 : 1; or ? 1 : 0;
to improve readability.
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20220511090006.90502-1-antoniu.miclaus@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/frequency/admv1014.c

@@ -700,8 +700,10 @@ static int admv1014_init(struct admv1014_state *st)
 			 ADMV1014_DET_EN_MSK;
 
 	enable_reg = FIELD_PREP(ADMV1014_P1DB_COMPENSATION_MSK, st->p1db_comp ? 3 : 0) |
-		     FIELD_PREP(ADMV1014_IF_AMP_PD_MSK, !(st->input_mode)) |
-		     FIELD_PREP(ADMV1014_BB_AMP_PD_MSK, st->input_mode) |
+		     FIELD_PREP(ADMV1014_IF_AMP_PD_MSK,
+				(st->input_mode == ADMV1014_IF_MODE) ? 0 : 1) |
+		     FIELD_PREP(ADMV1014_BB_AMP_PD_MSK,
+				(st->input_mode == ADMV1014_IF_MODE) ? 1 : 0) |
 		     FIELD_PREP(ADMV1014_DET_EN_MSK, st->det_en);
 
 	return __admv1014_spi_update_bits(st, ADMV1014_REG_ENABLE, enable_reg_msk, enable_reg);