drivers/linuxspidev: fix use of uninitialized speed variable

drivers/linuxspidev: fix use of uninitialized speed variable

Valgrind reported
== Syscall param ioctl(generic) points to uninitialised byte(s)
==    at 0x4ABF990: ioctl (ioctl.S:26)
==    by 0x19D00B: spidev_speed (linuxspidev.c:181)
==    by 0x19D00B: spidev_init (linuxspidev.c:307)

Indeed, spidev_init() uses adapter_get_speed(), it calls
adapter_khz_to_speed() and it returns early without setting
the output parameter if adapter is not initialized.
Of course the adapter initialized flag is not set until
spidev_init() returns.

Simply drop this code as the adapter infrastructure initializes
adapter speed just after spidev_init() return.

Change-Id: I26f011ae59fc942a34d9bb517f467c22f735091d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Fixes: 83e0293f7ba3 ("Add Linux SPI device SWD adapter support")
Reviewed-on: https://review.openocd.org/c/openocd/+/8733
Tested-by: jenkins
Reviewed-by: Richard Pasek <rpasek@google.com>