drivers/staging/speakup/devsynth.c: fix "buffer size is not provably correct" error

drivers/staging/speakup/devsynth.c: fix "buffer size is not provably correct" error

x86_64 allmodconfig:

In file included from arch/x86/include/asm/uaccess.h:572,
                 from include/linux/uaccess.h:5,
                 from drivers/staging/speakup/devsynth.c:4:
In function 'copy_from_user',
    inlined from 'speakup_file_write' at drivers/staging/speakup/devsynth.c:28:
arch/x86/include/asm/uaccess_64.h:64: error: call to 'copy_from_user_overflow' declared with attribute error: copy_from_user() buffer size is not provably correct

I'm not sure what was unprovable about it, but size_t is the correct type
anyway.

Also replace needless min_t() with min()

Cc: William Hubbs <w.d.hubbs@gmail.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>