tuner_e4k: fix off-by-one error in e4k_if_gain_set()

Thanks to patchvonbraun for providing debug logs
that helped to track this beast down.

Signed-off-by: Steve Markgraf <steve@steve-m.de>

src/tuner_e4k.c

@@ -715,7 +715,7 @@ int e4k_if_gain_set(struct e4k_state *e4k, uint8_t stage, int8_t value)
 		return rc;
 
 	/* compute the bit-mask for the given gain field */
-	field = &if_stage_gain_regs[stage];
+	field = &if_stage_gain_regs[stage-1];
 	mask = width2mask[field->width] << field->shift;
 
 	return e4k_reg_set_mask(e4k, field->reg, mask, rc << field->shift);