power: fix: Do not execute pmic command when not all necessary parameters are passed

Lack of this check resulted in a data abort when CPU tried to execute the
following command (without further mandatory input): 'pmic MAX77686_PMIC'.

Only the 'pmic list' command requires one passed parameter.
Other require at least two valid parameters for correct operation.
Signed-off-by: NLukasz Majewski <l.majewski@samsung.com>

drivers/power/power_core.c

@@ -140,6 +140,9 @@ int do_pmic(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 		return CMD_RET_SUCCESS;
 	}
 
+	if (argc < 3)
+		return CMD_RET_USAGE;
+
 	name = argv[1];
 	cmd = argv[2];