drivers: misc: ti-st: fix review comments

Based on comments from Jiri Slaby, drop the register
storage specifier, remove the unused code, cleanup
the const to non-const type casting.
Also make the line discipline ops structure static, since
its a singleton, unmodified structure which need not be
in heap.

Reported-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>