efi/libstub: Make efi_printk() input argument const char*

To help the compiler figure out that efi_printk() will not modify
the string it is given, make the input argument type const char*.

While at it, simplify the implementation as well.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>