serial: 8250_rsa: use guard()s

Having all the new guards, use them in the 8250_rsa code. This improves
readability, makes error handling easier, and marks locked portions of
code explicit.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250814072456.182853-14-jirislaby@kernel.org


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