sgi-gru: decrapfiy options_write() function

sgi-gru: decrapfiy options_write() function

Not a single line of actual code in the function was really
fundamentally correct.

Problems ranged from lack of proper range checking, to removing the last
character written (which admittedly is usually '\n'), to not accepting
hex numbers even though the 'show' routine would show the data in that
format.

This tries to do better.

Acked-by: Michael Buesch <[email protected]>
Tested-and-acked-by: Jack Steiner <[email protected]>
Cc: [email protected]
Cc: Jiri Kosina <[email protected]>
Cc: Michael Gilbert <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>