MAINTAINERS: serial:blackfin: update F: pattern

commit 0c6967b5 ("serial:blackfin: rename Blackfin serial driver to
bfin_uart.c") renamed the file, update the pattern.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>