iio: adc: at91_adc: allow to use full range of startup time

The DT-Property "atmel,adc-startup-time" is stored in an u8 for a microsecond
value. When trying to increase the value of STARTUP in Register AT91_ADC_MR
some higher values can't be reached.

Change the type in function parameter and private structure field from u8 to
u32.

Signed-off-by: Jan Leupold <[e-mail]>
[[e-mail]: change commit message, increase u16 to u32 for startup time]
Signed-off-by: Nicolas Ferre <[e-mail]>
Acked-by: Alexandre Belloni <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/at91_adc.c

···
 	u8	ts_pen_detect_sensitivity;
 
 	/* startup time calculate function */
-	u32 (*calc_startup_ticks)(u8 startup_time, u32 adc_clk_khz);
+	u32 (*calc_startup_ticks)(u32 startup_time, u32 adc_clk_khz);
 
 	u8	num_channels;
 	struct at91_adc_reg_desc registers;
···
 	u8			num_channels;
 	void __iomem		*reg_base;
 	struct at91_adc_reg_desc *registers;
-	u8			startup_time;
+	u32			startup_time;
 	u8			sample_hold_time;
 	bool			sleep_mode;
 	struct iio_trigger	**trig;
···
 	return ret;
 }
 
-static u32 calc_startup_ticks_9260(u8 startup_time, u32 adc_clk_khz)
+static u32 calc_startup_ticks_9260(u32 startup_time, u32 adc_clk_khz)
 {
 	/*
 	 * Number of ticks needed to cover the startup time of the ADC
···
 	return round_up((startup_time * adc_clk_khz / 1000) - 1, 8) / 8;
 }
 
-static u32 calc_startup_ticks_9x5(u8 startup_time, u32 adc_clk_khz)
+static u32 calc_startup_ticks_9x5(u32 startup_time, u32 adc_clk_khz)
 {
 	/*
 	 * For sama5d3x and at91sam9x5, the formula changes to: