net: unaligned copying of unsigned long

The inline functions net_read_u32() and net_copy_u32() have been created to
copy unaligned u32. But this is not obvious to the compiler. GCC 9.1
introduces a check -Werror=address-of-packed-member which leads to a build
error on Travis CI:

net/bootp.c: In function ‘dhcp_send_request_packet’:
net/bootp.c:1011:27: error: taking address of packed member of
‘struct bootp_hdr’ may result in an unaligned pointer value
[-Werror=address-of-packed-member]
 1011 |  net_copy_u32(&bp->bp_id, &bp_offer->bp_id);

Change the type of the function parameters to void * to avoid the build
error.
Reported-by: NRamon Fried <rfried.dev@gmail.com>
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: NJoe Hershberger <joe.hershberger@ni.com>

include/net.h

@@ -728,7 +728,7 @@ static inline struct in_addr net_read_ip(void *from)
 }
 
 /* return ulong *in network byteorder* */
-static inline u32 net_read_u32(u32 *from)
+static inline u32 net_read_u32(void *from)
 {
 	u32 l;
 
@@ -749,7 +749,7 @@ static inline void net_copy_ip(void *to, void *from)
 }
 
 /* copy ulong */
-static inline void net_copy_u32(u32 *to, u32 *from)
+static inline void net_copy_u32(void *to, void *from)
 {
 	memcpy((void *)to, (void *)from, sizeof(u32));
 }