efi/printf: Fix minor bug in precision handling

A negative precision should be ignored completely, and the presence of a
valid precision should turn off the 0 flag.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-10-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>