scripts: dtc: fix compile warnings

Fix following compile warnings:

scripts/dtc/flattree.c: In function ‘flat_read_mem_reserve’:
scripts/dtc/flattree.c:700:14: warning: variable ‘p’ set but not used
	[-Wunused-but-set-variable]

scripts/dtc/dtc.c: In function ‘main’:
scripts/dtc/dtc.c:104:17: warning: variable ‘check’ set but not used
	[-Wunused-but-set-variable]

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>