fastboot: getvar: correct/rename "has_slot" to "has-slot"

Since its inception in upstream fastboot android-n-preview-1 [1],
"has-slot" option has never taken the form of "has_slot". Amongst the
users of "getvar has-slot:" is the upstream bootloadertest.py [2].

Current U-Boot "has_slot" version must be a typo. Fix it.

[1] https://android.googlesource.com/platform/system/core/+/a797479bd51c
    ("Fix fastboot variable name")
[2] https://android.googlesource.com/platform/system/extras/+/72de393e118e3
    ("Bootloader verification for AndroidThings.")

Fixes: f73a7df9 ("net: fastboot: Merge AOSP UDP fastboot")
Signed-off-by: NEugeniu Rosca <erosca@de.adit-jv.com>
Acked-by: NAlex Kiernan <alex.kiernan@gmail.com>

drivers/fastboot/fb_getvar.c

@@ -62,7 +62,7 @@ static const struct {
 		.variable = "slot-suffixes",
 		.dispatch = getvar_slot_suffixes
 	}, {
-		.variable = "has_slot",
+		.variable = "has-slot",
 		.dispatch = getvar_has_slot
 #if CONFIG_IS_ENABLED(FASTBOOT_FLASH_MMC)
 	}, {