greybus: es2: Fix 'make check' warnings with arpc

This is what I got:

greybus/es2.c:1130:29: warning: restricted __le16 degrades to integer
greybus/es2.c:1140:22: warning: incorrect type in assignment (different base types)
greybus/es2.c:1140:22:    expected restricted __le16 [usertype] id
greybus/es2.c:1140:22:    got unsigned short [unsigned] [usertype] <noident>
greybus/es2.c:1162:52: warning: incorrect type in argument 8 (different base types)
greybus/es2.c:1162:52:    expected unsigned short [unsigned] [usertype] size
greybus/es2.c:1162:52:    got restricted __le16 [usertype] size
greybus/es2.c:1164:31: warning: restricted __le16 degrades to integer
greybus/es2.c:1253:34: warning: incorrect type in argument 2 (different base types)
greybus/es2.c:1253:34:    expected unsigned char [unsigned] [usertype] id
greybus/es2.c:1253:34:    got restricted __le16 [usertype] id

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Alex Elder <elder@linaro.org>