drivers/mtd/cfi_flash.c: sparse fixes

cfi_flash.c:756:43: warning: Using plain integer as NULL pointer
cfi_flash.c:1150:52: warning: cast to non-scalar
cfi_flash.c:1433:46: warning: cast truncates bits from constant value (890089 becomes 89)
cfi_flash.c:1490:61: warning: cast truncates bits from constant value (1f001f becomes 1f)
cfi_flash.c:1508:61: warning: cast truncates bits from constant value (10001 becomes 1)
cfi_flash.c:1738:63: warning: cast truncates bits from constant value (10001 becomes 1)
cfi_flash.c:1857:6: warning: symbol '__flash_cmd_reset' was not declared. Should it be static?

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>