usb: gadget: serial: remove PREFIX macro

Remove the ttyGS PREFIX macro from u_serial.c and replace all occurences with
the hardcoded ttyGS string.

This macro was mostly used in a few debug/warning messages and a lot of
hardcoded ttyGS existed beneath. It may have been used for renaming the
tty, but if done so most debug messages would have ignored this.

Due to the fact the usage of this PREFIX in all debug calls would have
resulted in a hard to read/grep code it is removed completely.

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>