mtd: maps: pcmciamtd: fix possible sleep-in-atomic-context bugs in pcmciamtd_set_vpp()

The driver may sleep while holding a spinlock.
The function call path (from bottom to top) in Linux 4.19 is:

drivers/pcmcia/pcmcia_resource.c, 312:
	mutex_lock in pcmcia_fixup_vpp
drivers/mtd/maps/pcmciamtd.c, 309:
	pcmcia_fixup_vpp in pcmciamtd_set_vpp
drivers/mtd/maps/pcmciamtd.c, 306:
	_raw_spin_lock_irqsave in pcmciamtd_set_vpp

drivers/pcmcia/pcmcia_resource.c, 312:
	mutex_lock in pcmcia_fixup_vpp
drivers/mtd/maps/pcmciamtd.c, 312:
	pcmcia_fixup_vpp in pcmciamtd_set_vpp
drivers/mtd/maps/pcmciamtd.c, 306:
	_raw_spin_lock_irqsave in pcmciamtd_set_vp

mutex_lock() may sleep at runtime.

To fix these bugs, the spinlock is replaced with a mutex.

These bugs are found by a static analysis tool STCheck written by
myself.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>