[PATCH] zd1211rw: Readd zd_addr_t cast

Robert P.J. Day's recent commit ("getting rid of all casts of
k[cmz]alloc() calls") introduced a sparse warning for zd1211rw,
related to our type-checking of addresses.

	zd_chip.c:116:15: warning: implicit cast to nocast type

This patch readds the type cast, it is correct.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>