i2c: mvtwsi.c: Fix set speed

Previous patch for this driver breaks i2c initialization.

commit 8bcf12cc ("i2c: mvtwsi.c: Avoid NULL dereference")

If actual_speed is passed as NULL in this function:
static void __twsi_i2c_init(struct mvtwsi_registers *twsi, int speed,
			    int slaveadd, uint *actual_speed)
than __twsi_i2c_set_bus_speed never get called. This causes i2c clock
to run on default speed - 2MHz (measured with oscilloscope). This is issue
on some boards, sunxi for example, since on I2C0 bus PMU is connected.

The bootlogs with and without the patch are as follows:

Wihtout the patch:
U-Boot SPL 2018.03-rc2 (Feb 13 2018 - 09:23:17 +0200)
DRAM: 1024 MiB
Failed to set core voltage! Can't set CPU frequency
Trying to boot from FEL

U-Boot 2018.03-rc2 (Feb 13 2018 - 09:23:17 +0200) Allwinner Technology

CPU:   Allwinner A20 (SUN7I)
Model: Olimex A20-OLinuXino-LIME2
I2C:   ready
DRAM:  1 GiB
MMC:   SUNXI SD/MMC: 0

With the patch:
U-Boot SPL 2018.03-rc2-00001-g838ff85 (Feb 13 2018 - 09:24:34 +0200)
DRAM: 1024 MiB
CPU: 912000000Hz, AXI/AHB/APB: 3/2/2
Trying to boot from FEL

U-Boot 2018.03-rc2-00001-g838ff85 (Feb 13 2018 - 09:24:34 +0200) Allwinner Technology

CPU:   Allwinner A20 (SUN7I)
Model: Olimex A20-OLinuXino-LIME2
I2C:   ready
DRAM:  1 GiB
MMC:   SUNXI SD/MMC: 0

Signed-off-by: Stefan Mavrodiev <stefan@olimex.com>
Reviewed-by: Heiko Schocher <hs@denx.de>