drivers/serial: Change the default value of SERIAL_NPOLLWAITERS to 4

since the console is normally opened three time as stdin, stdout and
stderr and remove the dependence on STANDARD_SERIAL because other type
of serial drivers also support the poll through the same codebase.

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Ie68322c4647cc838b295491329969869d6ba310b

drivers/serial/Kconfig

@@ -91,11 +91,10 @@ config STANDARD_SERIAL
 
 config SERIAL_NPOLLWAITERS
 	int "Number of poll threads"
-	default 2
-	depends on STANDARD_SERIAL
+	default 4
 	---help---
 		Maximum number of threads than can be waiting for POLL events.
-		Default: 2
+		Default: 4
 
 config SERIAL_IFLOWCONTROL
 	bool

include/nuttx/serial/serial.h

@@ -45,7 +45,7 @@
 /* Maximum number of threads than can be waiting for POLL events */
 
 #ifndef CONFIG_SERIAL_NPOLLWAITERS
-#  define CONFIG_SERIAL_NPOLLWAITERS 2
+#  define CONFIG_SERIAL_NPOLLWAITERS 4
 #endif
 
 /* RX flow control */