General help message cleanup

Many of the help messages were not really helpful; for example, many
commands that take no arguments would not print a correct synopsis
line, but "No additional help available." which is not exactly wrong,
but not helpful either.

Commit ``Make "usage" messages more helpful.'' changed this
partially. But it also became clear that lots of "Usage" and "Help"
messages (fields "usage" and "help" in struct cmd_tbl_s respective)
were actually redundant.

This patch cleans this up - for example:

Before:
	=> help dtt
	dtt - Digital Thermometer and Thermostat

	Usage:
	dtt         - Read temperature from digital thermometer and thermostat.

After:
	=> help dtt
	dtt - Read temperature from Digital Thermometer and Thermostat

	Usage:
	dtt

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/drivers/gpio/pca953x.c b/drivers/gpio/pca953x.c
index 1c5e314..e8273ee 100644
--- a/drivers/gpio/pca953x.c
+++ b/drivers/gpio/pca953x.c
@@ -221,7 +221,7 @@
 	"pca953x invert pin 0|1\n"
 	"	- disable/enable polarity inversion for reads\n"
 	"pca953x intput pin\n"
-	"	- set pin as input and read value\n"
+	"	- set pin as input and read value"
 );
 
 #endif /* CONFIG_CMD_PCA953X */