tcl: don't use 'set' to retrieve the value of a variable

tcl: don't use 'set' to retrieve the value of a variable

Original TLC syntax uses 'set varname' to retrieve the value of
variable 'varname'. Such archaic syntax is still valid, but the
shorter '$varname' makes the code easier to read.

Replace 'set varname' with '$varname'.
While there, remove some useless curly brackets.

Change-Id: I27310e8c05afe56ea8bd0e41d4ae2c34447b725c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6863
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>