amiserial: expand serial_isroot

Having a macro (serial_isroot) for capable(CAP_SYS_ADMIN) does not save
us from anything. It rather obfuscates the code. Hence expand
serial_isroot to be explicit like every other driver is.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210714091314.8292-7-jslaby@suse.cz


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