SPI Kconfig simplifications

Use "if SPI_MASTER" to remove numerous dependencies.

[dbrownell@users.sourceforge.net: remove a couple now-needless EXPERIMENTAL dependencies too]
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>