nor/at91sam4: change prototype of get_current_sam4()

nor/at91sam4: change prototype of get_current_sam4()

To prepare for handling TCL return values consistently, all calls
to command_print/command_print_sameline should be ready to switch
to CMD as first parameter.

Change prototype of get_current_sam4() to pass CMD instead of
CMD_CTX.

This change was part of http://openocd.zylin.com/1815 from Paul
Fertser and has been extracted and rebased to simplify the review.

Change-Id: I8dfa66f3f6be318d6ba89649279b1b4502f375d9
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5055
Tested-by: jenkins