PATI board: fix compiler warnings

Fix these:
pati.c: In function 'checkboard':
pati.c:358: warning: pointer targets in passing argument 2 of 'getenv_r' differ in signedness
../common/flash.c: In function 'write_word':
../common/flash.c:824: warning: dereferencing type-punned pointer will break strict-aliasing rules
cmd_pati.c: In function 'do_pati':
cmd_pati.c:279: warning: 'value' may be used uninitialized in this function

Signed-off-by: Wolfgang Denk <wd@denx.de>