tty/vt: consolemap: make conv_uni_to_pc() more readable

1) Fetch *conp->vc_uni_pagedir_loc first and do the NULL check on the local
   variable.
2) Decouple the large "if" into few smaller "if"s.
3) Remove a \n from the definition line.

This makes the code more readable.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220607104946.18710-31-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>