Currently gpmc_cs_reserved() return value is somewhat inconsistent,
returning a negative value on an error condition, a positive value
if the chip select is reserved and zero if it's available.

Fix this by returning a boolean value as the function name suggests:
  * true if the chip select is reserved,
  * false if it's available
Suggested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>