uvesafb: improve parameter handling.

1) Now module_param(..., invbool, ...) requires a bool, and similarly
   module_param(..., bool, ...) allows it, change pmi_setpal to a bool.
2) #define param_get_scroll to NULL, since it can never be called (perm
   argument to module_param_named is 0).
3) Return -EINVAL from param_set_scroll if the value is bad, so it's
   reported.

Note that I don't think the old fb_get_options() is required for new
drivers: the parameters automatically work as uvesafb.XXX=... anyway.

Acked-by: Michał Januszewski <spock@gentoo.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>