esp6: remove redundant initialization of esph

The pointer esph is being initialized with a value that is never
read and then being updated.  Remove the redundant initialization
and move the declaration and initializtion of esph to the local
code block.

Cleans up clang warning:
net/ipv6/esp6.c:562:21: warning: Value stored to 'esph' during its
initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>