n_gsm: Use print_hex_dump_bytes

Use the standard mechanism to print a hex buffer
to eliminate empty printf warning.

A couple % smaller text and data too.

$ size drivers/tty/n_gsm.o*
   text	   data	    bss	    dec	    hex	filename
  23543	    312	   6376	  30231	   7617	drivers/tty/n_gsm.o.new
  24051	    408	   6496	  30955	   78eb	drivers/tty/n_gsm.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>