tty: sysrq: use switch in sysrq_key_table_key2index()

Using switch with range cases makes the code more aligned and readable.
Expand also that 36 as explicit addition of 10 + 26 to make the source
of the constant more obvious.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230712081811.29004-5-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>